Method for conducting sensor array-based rapid materials characterization

ABSTRACT

A materials characterization method uses a sensor array disposed on a substrate, with an array and contact pad; electronic test and measurement apparatus for sending electrical signals to and receiving electrical signals from the sensor array; an apparatus for making electrical contact to the sensors in the standardized array format; and an apparatus for routing signals between one or more selected sensors and the electronic test and measurement apparatus. The method comprises applying multiple material samples to the multiple sensors in the array; electrically contacting one or more sensors in the array; making electrical connections between selected sensors and the electronic test and measurement apparatus; and sending and receiving signals to and from the sensors in the array, where the electrical signals correspond to the thermal, electrical, mechanical, or other properties of the material samples. The sensor array is preferably arranged in a standardized format used in combinatorial chemistry applications for rapid deposition of sample materials on the sensor array. The standardized interconnection apparatus and standardized sensor array and contact pad format allow measurement of many different material properties by using substrates carrying different sensor types, with only minor modifications if any to the electronic test and measurement apparatus and test procedures. By using a sensor array that is separate from the electronic apparatus, and by including standardized contacting and signal routing apparatuses, the method takes advantage of a modular “plug-and-play” system that eliminates the need for multiple materials characterization machines, and eliminates the need for application-specific active circuitry within the sensor arrays themselves. Further, the method can characterize large numbers of material samples rapidly, on the order of at least 50 samples per hour, reducing the time needed for screening of materials libraries.

RELATED CASES

[0001] The present application is related to co-pending U.S. patentapplication Ser. No. 09/______ (Attorney Docket No. 65304-39/SYMYX98-23) and U.S. patent application Ser. No. 09/______ (Attorney DocketNo. 65304-055/SYMYX 98-36), all filed on Dec. 10, 1998 and which areincorporated herein by reference.

TECHNICAL FIELD

[0002] The present invention is directed to a method for characterizinga plurality of organic or inorganic materials, and more particularly toa characterization method that uses a modular, electrically-drivensensor array in a selected standardized integrated electronic platformto characterize a plurality of materials simultaneously and rapidly.

BACKGROUND

[0003] Companies are turning to combinatorial materials sciencetechniques for developing new compounds or materials (includingformulations, materials having different processing histories, ormixtures of compounds) having novel physical and chemical properties.Combinatorial materials science refers generally to methods andapparatuses for creating a collection of chemically diverse compounds ormaterials and to methods and apparatuses for rapidly testing orscreening such compounds or materials for desired performancecharacteristics and/or properties. The collections of chemical compoundsor materials are commonly called “libraries”. See U.S. Pat. No.5,776,359, herein incorporated by reference, for a general discussion ofcombinatorial methodologies.

[0004] A virtually infinite number of useful materials or compounds canbe prepared by combining different elements of the Periodic Table ofElements in varying ratios, by creating compounds with differentarrangements of elements, and by creating materials comprising mixturesof compounds or formulations with differing processing histories.Discovery of useful materials for a particular application may requirepreparation or characterization of many candidate materials orcompounds. Preparing and screening a large number of candidatesincreases the probability of useful discoveries. Thus, any system thatcan analyze and characterize the properties of combinatorially preparedlibrary members quickly and accurately is highly desirable.

[0005] Many conventional measurement systems comprise a distinctspecialized machine for characterizing a particular material property,so that testing of a candidate material can use many machines and becumbersome and time-consuming. Also, most known materialscharacterization devices measure only one material sample at a time,severely limiting the number of samples that can be characterized perunit time.

[0006] Optical screening methods and devices have been preferred formany combinatorial chemistry and combinatorial materials scienceapplications because they are non-contact and non-destructive. See forexample WO 98/15805, incorporated herein by reference. For example,luminescence may be screened optically. When monitoring chemicalreactions, for example, thermal imaging with an infrared camera candetect heat released during relatively fast exothermic reactions. See WO98/15813, incorporated herein by reference. Although optical methods areparticularly useful for characterizing materials or properties incertain circumstances, many materials characterization techniques aredifficult or impossible to perform using optical methods. Therefore,there is still a need for a more direct materials characterizationmethod that involves more intimate contact between the material samplesand the sensing apparatus.

[0007] Conventional sensors that generate electrical data correspondingto material properties are typically designed as individual, discreteunits, each sensor having its own packaging and wiring connections. Manymaterials characterization sensors are designed to be used individuallyin or with a machine that characterizes one sample at a time. Linking aplurality of these individual sensors in an array format, assuming thatit is physically possible, would be expensive and often creates overlycomplicated wiring schemes with minimal gains in operating efficiencyfor the overall sensing system.

[0008] One structure using multiple material samples is amicrofabricated array containing “microhotplates”. The microhotplatesact as miniature heating plates for supporting and selectively heatingmaterial samples placed thereon. U.S. Pat. No. 5,356,756 to Cavicchi etal and U.S. Pat. No. 5,345,213 to Semancik et al. as well the articleentitled “Kinetically Controlled Chemical Sensing Using MicromachinedStructures,” by Semancik and Cavicchi, (Accounts of Chemical Research,Vol. 31, No. 5, 1998), all illustrate the microhotplate concept and areincorporated herein by reference. Although arrays containingmicrohotplates are known, they have been used primarily to create variedprocessing conditions for preparing materials. A need still exists foran array-based sensor system that can actually characterize materialproperties.

[0009] It is therefore an object of the invention to provide a materialscharacterization system that can measure properties of many materialsamples quickly, and in some embodiments simultaneously.

[0010] It is also an object of the invention to construct a materialscharacterization system having a modular structure that can be connectedto a flexible electronic platform to allow many different materialproperties to be measured with minimal modification of the apparatus.

SUMMARY OF THE INVENTION

[0011] This invention provides an apparatus (or system) and method fortesting materials in an array format using sensors that contact thematerials being tested. Accordingly, the present invention is directedto an electronically-driven sensor array system for rapidcharacterization of multiple materials. A plurality of sensors aredisposed on a substrate to form a sensor array. Properties that can bemeasured include thermal, electrical and mechanical properties ofsamples. Regardless of the property being measured or the specificapparatus, the materials characterization system of the inventionincludes a multiple sensors carrying multiple samples, means for routingsignals to and from the sensors, electronic test circuitry, and acomputer or processor to receive and interpret data from the sensors. Ina preferred embodiment, a modular system is constructed including asingle sensor array format, and signal routing equipment compatible withthis format which can be used with multiple sensor types and multipleelectronic test equipment types, permitting maximum flexibility of thesystem while preserving the general advantages of sensor array-basedcharacterization. Alternatively, some or all of the different parts ofthe system may be integrated together into a single physical componentof the system.

[0012] The sensors can be operated in serial or parallel fashion. A widerange of electronically driven sensors may be employed, which those ofskill in the art will appreciate provide the opportunity to design anapparatus or method for specific applications or property measurements.The environment in which the measurement is made by the sensor can becontrolled.

[0013] This invention allows for rapid screening of combinatoriallibraries or large numbers of samples prepared by other means. Thisinvention allows for property measurements that cannot be doneoptically. However, optical measurements may be made in conjunction withthe sensor based electronic measurements of this invention. Onepotentially important feature is the speed of the property measurementsmade with this invention. Two independent reasons for this speed arethat one can measure samples in parallel or with smaller sample sizesthan with conventional measurement techniques. Moreover, automatedsample handling, array preparation and/or sensor operation allows for acompletely automated rapid property measurement system in accord withthis invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIGS. 1A through 1E are diagrams illustrating the overall systemof the present invention;

[0015]FIG. 2A through 2D are diagrams illustrating examples of sensorarray and contact configurations in the present invention;

[0016]FIGS. 3A and 3B are examples of a printed circuit board in theinvention;

[0017]FIG. 4 is one embodiment of a sensor array/circuit board assemblyin the invention;

[0018]FIG. 5 is a representative diagram of a matrix switch in theinvention;

[0019]FIGS. 6A and 6B are representative diagrams illustrating twocontemplated sensor addressing schemes in the invention;

[0020]FIG. 7 illustrates one alternative contact structure for thesensor array;

[0021]FIG. 8 illustrates another embodiment of the invention;

[0022]FIGS. 9A through 9C are examples of a sensor structure for thermalanalysis in the present invention;

[0023]FIG. 10 illustrates an alternative thermal analysis sensorsubstrate structure;

[0024]FIGS. 11A through 11F are sample traces of thermal analysis scansconducted according to the present invention;

[0025]FIGS. 12A through 121 illustrate one system for conducting thermalanalysis according to the present invention;

[0026]FIGS. 13A through 13G illustrate another system for conductingthermal analysis according to the present invention;

[0027]FIG. 14 illustrates a thermal decomposition measurement accordingto the invention;

[0028]FIGS. 15A and 15B illustrate dynamic thermal analysis conductedaccording to the present invention;

[0029]FIGS. 16A through 16E illustrate dielectric spectroscopy conductedaccording to the present invention;

[0030]FIGS. 17A and 17B show an example of a mechanical resonatorstructure that can be used in the invention;

[0031]FIGS. 18A through 18C illustrate electrical transportcharacterization conducted according to the present invention;

[0032]FIGS. 19A through 19C illustrate thermoelectric propertycharacterization conducted according to the present invention;

[0033]FIGS. 20A and 20B illustrate thermal conductivity characterizationconducted according to the present invention; and

[0034]FIGS. 21A and 21B illustrate magnetic property characterizationconducted according to the present invention.

DETAILED DESCRIPTION

[0035]FIG. 1A illustrates the generic apparatus or system concept of thematerials characterization system of the present invention, and FIGS. 1Bthrough 1E illustrates possible variations of the system. Regardless ofthe property being measured or the specific hardware in the apparatus,the materials characterization system of the invention includes multiplesensors in contact with multiple samples, means for routing signals toand from the sensors, electronic test circuitry, and a computer orprocessor to receive and interpret data from the sensors or theelectronic test circuitry. FIG. 1B is a representative diagram of anapparatus where each component is separate and interchangeable, allowingmaximum flexibility and interchangeability of parts. FIGS. 1C through 1Eillustrate variations where portions of the apparatus, such as thesensor array and electronic test circuitry, are integrated into onepart, allowing for a more compact design, but with a greater degree ofcustomization of the apparatus for a particular application or propertymeasurement. Regardless of the degree to which components in theapparatus are integrated into one unit, the overall operation of thesensor-array based apparatus remains the same, as will be explained infurther detail below.

[0036] In one embodiment that those of skill in the art will appreciateprovides a great deal of flexibility, each sensor has adjacent to it aplurality of associated sensor contact pads. Alternatively, the contactpads can be arranged near the edges of the sensor array, with leads onthe substrate connecting the sensors to the contact pads, to prevent thecontact pads from being contaminated with the materials being tested.The system in this embodiment also includes a printed circuit boardhaving a plurality of board contact pads arranged in the sameconfiguration as the sensor contact pads in the sensor array.Connectors, such as conducting elastomers, stick probes, cantileverprobes, conducting adhesives, wafer-to-board bonding techniques, orother contact devices, couple the sensor array with the printed circuitboard by creating contacts between the sensor contact pads and the boardcontact pads, preferably the contacts are reversible and non-permanent.Thus, sensor arrays incorporating different sensor functionalities canbe created using the same array and contact pad format and contactedusing the same circuit board and connections.

[0037] The printed circuit board in the inventive system also includestraces that connect the individual contact pads to standard multi-pinconnectors placed near the edges of the board. This construction allowseasy connection between the printed circuit board assembly and the restof the system using standard multi-wire ribbon cable assembliescompatible with the chosen multi-pin connectors. In the system accordingto a preferred embodiment, the multiwire cables and connectors couplethe printed circuit board assembly to a multiplexer or other signalrouting means for selecting one or more sensors to be activated,depending on the specific software instructions to the signal routingmeans. The multiplexer or signal routing means is, in turn, coupled to aflexible electronic platform, which can include electronic test andmeasurement circuitry, a computer, or both. The electronic platform canalso include a switch matrix, preferably under control of the computer,for connecting the multiplexer outputs to a variety of differentelectronics test instruments without manually reconnecting cables. Thus,when a sensor array incorporating different sensor functionality isneeded, to test for a different material property, only minimalreconfugration of the electronic platform is needed. In this manner, thesame system can be used to test for a wide variety of materialproperties.

[0038] In other cases, it may be desirable to collect information frommany sensors simultaneously, rather than in a rapid serial fashion. Inthe preferred embodiment of the invention for such cases, the multi-wirecables and connectors themselves serve as the signal routing means andare directly attached to an electronics module having a multiplicity ofindependent electronics channels for driving and reading the sensors.The outputs of these independent channels are then collected by thecomputer.

[0039] The sensor array itself may contain different types of sensorsdesigned to measure different material properties in the differentoperation modes as well. Further, standardizing the sensor arrayconfiguration, the contact format, and the connections from the board tothe multiplexer and/or the electronic platform allows easy“plug-and-play” interconnection as well as simplification of the sensorstructures themselves. In one embodiment of the invention, no activecircuitry is included in the sensor array, reducing the manufacturingcost of the sensor array enough to make the sensor array disposable, ifdesired.

[0040] In a preferred embodiment, the sensor array has the same formatas a standardized format used in combinatorial chemistry applications(e.g., an 8×12 grid with 9 mm spacing in between each sensor). By usinga standardized format, substances to be tested by the sensors incombinatorial applications can be placed on multiple sensorssimultaneously rather than one sensor at a time, e.g., via simultaneoustransfer from a standard microtiter plate, further increasing testingand processing speed in the apparatus. The sensors in a single array canbe constructed so that they all measure the same material property, oralternatively a single array can contain several different types ofsensors that measure different material properties. The modular formatof the sensor array, the standardized interconnection means, and theflexible electronic platform allows a great deal of flexibility indetermining what types of sensors to include in the array since the samegeneral electronic platform (e.g. electronic test circuitry andcomputer) and array format is used, regardless of the specific propertybeing measured.

[0041] Alternatively, the sensors can be suspended at the end of anarray of rods or plates that hang vertically from a common supportingplate, preferably in a standard combinatorial chemistry format, to forma “dipstick” array structure. The sensors can then be dippedsimultaneously into wells containing solutions of materials to becharacterized. After removing the dipsticks from the solutions andallowing the solvent to evaporate, the sensors are coated with a film ofmaterial. The material can then be tested in the same manner as thesensors in the flat sensor array. The materials or liquids can also betested while in the wells. Other embodiments of the invention includeintegrating the printed circuit board with the signal routing partsand/or the electronic test circuitry to construct a more customizedcharacterization device or placing all components and electroniccircuitry on the same substrate as the sensors.

[0042]FIGS. 2A, 2B and 2D illustrate one example of a sensor array andcontact pad layout pattern using an 8×8 square array with a 0.25-inchpitch (spacing between the centers of adjacent sensors in the array).This particular two-inch square sensor array is compatible with vapordeposition chamber equipment that is often used in combinatorialchemistry and combinatorial materials science applications.

[0043] Another widely used combinatorial configuration is an 8×12rectangular array with a 9 mm pitch, shown in FIG. 2C. The specificsensor array configuration is selected to be compatible with, forexample, the automated deposition equipment being used and/or thephysical configuration of the material libraries being tested. Astandardized sensor array configuration allows material depositionapparatus to deposit entire rows, columns or an entire library ofsamples on all of the sensors in the array simultaneously, which isgenerally more efficient than depositing materials one sensor at a time.The specific material deposition method used depends on the materialproperties being measured and the physical characteristics of thematerial itself. Automated dispensing systems may also be used, whichare well known to those of skill in the art, for example see U.S. Pat.No. 5,104,621. For example, in some thermal analyses, it is desirable todissolve the material to be characterized in a solvent, deposit solutiononto the sensor, and let the solvent evaporate to leave a film ofmaterial on the sensor's surface. For other materials, it may be moreappropriate to place material that is in the form of a slurry or powderdirectly on the sensor. Sample thickness on the sensor may depend on thetesting method, the sample itself or the method of sample deposition.Throughout this specification, the terms “thin” and “thick” may be usedwhen referring to films, however, those terms are not meant to belimiting.

[0044] Referring to FIGS. 2A and 2B, the sensor array 10 includes aplurality of sensors 12 and a plurality of sensor contact pads 14corresponding to the sensors 12. The specific micro-structure of thesensor 12 depends on the material property or properties that the sensor12 is designed to measure. Sensors 12 that are designed to measuredifferent properties have different micro-structures. More detaileddescriptions of the actual sensor 12 structure are provided below in theExperimental Example sections with respect to sensors that measurespecific material properties. To the naked eye, however, the sensors 12may look like small pads or tiny wells, depending on the specificmaterial characterization application, that are arranged on a planarsubstrate 16; the functional differences are within each individualsensor 12 at the microscopic level. More importantly, different sensorarrays 10, incorporating different sensors 12, will share a common array10 and contact pad 14 format. The electronic wiring and interconnectiondevices for sending sensor data to and from the sensor array 10 arearranged into a configuration that is compatible with the sensor array10 format. As a result, different sensor arrays 10 for use in the samematerials characterization apparatus will have the same sensor locationsand the same overall wiring patterns for electrical connections;different arrays 10 will look identical at a superficial level, even ifthey measure different properties. This sensor array 10 standardizationallows arrays 10 that measure completely different material propertiesto be electrically contacted using a single interconnection device,which is in turn attached to a flexible electronic platform.

[0045] The sensors 12 and sensor contact pads 14 are formed on thesubstrate 16 in any selected array format that is desired. For example,they may be compatible with the material deposition machine being used.Any desired geometry can be achieved, such as lines, squares,rectangles, circles, triangles, spirals, abstract shapes, etc. Suchgeometric shapes can be considered to have either an open or closedshape with either straight or curved sides or both. Any number ofsensors 12 can be used, including 5 sensors, 48, 96 or 128 sensors, andpreferably from 5 to 400 sensors may be in one array 10. The materialselected for the substrate 16 can vary depending on the application inwhich the sensor array 10 will be used, as will be explained by examplesbelow. Possible substrate materials include, but are not limited to,silicon, silicon nitride, glass, amorphous carbon, quartz, sapphire,silicon oxide or a polymer sheet. For example, the polymer substrate maybe a polyimide such as Kapton® from DuPont. Other polymer substrates maybe used, including those selected from the group consisting of aramids(such as Kevlar®), polyester (such as poly(ethyleneterephthalate),oriented films such as Mylar®, or poly(ethylenenaphthalate)), epoxyresins, phenol-formaldehyde resins, polytetrafluoroethylene (such asTeflon®), polyacetal (such as Delrin®), polyamide (such as Nylon®),polycarbonates, polyolefins, polyurethanes, silicones, polysiloxanes andthe like. Other materials can also be used for the substrate 16.Substrates 16 used for thermal characterization and other testsrequiring thermal isolation of small amounts of sample material shouldhave the ability to be formed into a thin film or sheet that canwithstand the temperatures at which the materials will be tested. In athermal analysis application in which the sample is a thin film, forexample, the portion of the substrate 16 that supports the sample isideally between 0.1 and 25 micrometers thick or the same order ofmagnitude as the thickness of a material sample, to minimize the effectsof the heat capacity and thermal conductivity of the substrate 16 in thetest results without making the substrate 16 too fragile to work witheasily. In short, the optimum dimensions of the substrate 16 will dependon the characteristics of the specific material chosen for the substrate16 and the specific property or properties to be characterized by thesensor array 10.

[0046] The sensors 12 and sensor contact pads 14 are preferably formedon the substrate 16 via lithography. The specific number and design ofthe lithographic layers will depend on the characteristics to bemeasured and the particular sensor application. If possible, the numberof layers is preferably as few as possible, for example less than fouror five layers, to minimize the number of fabrication steps and reducethe overall cost of the sensor array. The number of lithographic layerscan be kept to a minimum by creating sensors 12 that characterize onlyone or two material properties and also by eliminating on-board controlcircuitry within the sensor 12 itself, if desired. More specific sensorstructures are explained in further detail below with respect to theexperimental examples. Keeping the sensor array 10 manufacturing costlow makes disposability of the array 10 possible, if desired ornecessary (e.g. after testing inorganic materials that may not be easilyremoved from the sensors). Further, if there is no onboard controlcircuitry that could be harmed under extreme conditions on the sensorarray 1O, the sensor array 10 can be cleaned after use by dipping theentire array structure into a solvent or acid or heating the sensorarray 10 at a very high temperature to remove sample material residue.The cleaned sensor array 10 can then be reused. Of course, placingon-board electronics on the sensor array 10 or integrating the arraywith a circuit board having electronic components is also an option, ifdeemed appropriate for the application in which the array 10 will beused.

[0047] In one embodiment, eight sensor contact pads 14 are provided foreach sensor, as shown in FIG. 2A, 2B and 2D. For identificationpurposes, the eight pads can be divided into four pairs labelled Athrough D, with each pair having a H (high) contact pad and a L (low)contact pad. Using this labelling scheme, each sensor contact pad in thesensor array can be identified by an array position, a letter, and a Hor L designation (e.g., (1,1)AH). Of course, other sensor 12 and sensorcontact pad 14 configurations are possible as well as alternative sensorcontact pad identification systems. Also, in this example, the sensorcontact pads 14 are preferably spaced at a {fraction (1/16)} inch pitchwith a 1 mm spacing in between adjacent columns of pads 14. Thisphysical arrangement is particularly suited for coupling the sensorcontact pads 14 to a printed circuit board 30 via elastomericconnectors, which will be explained in greater detail below. Othersensor contact pad 14 arrangements can also be used, depending on thespecific manner in which the sensor array 10 will be electricallycontacted and the specific application in which the sensor array 10 willbe used, without departing from the scope of the invention.

[0048]FIGS. 3A and 3B are top views of a specific embodiments of printedcircuit board to be coupled with the sensor array 10 shown in FIG. 2Aand 2B, and FIG. 4 shows an exploded view of one portion of an apparatusthat connects the sensor array to the circuit board in the inventivematerials characterization device. The circuit board used in theexamples (except for the dielectric example) measures 11 inches indiameter and includes 8 layers of metallization. Gold was used for thetop layer of metallization to for good electrical contact withelastomeric connectors. All eight layers are super-imposed in FIG. 3B.Of course, this specific design can be modified by those of skill in theart without departing from the invention. Generally, the printed circuitboard 30 preferably includes a plurality of board contact pads 32 havingan arrangement which is a mirror image of the arrangement of the sensorarray contact pads 14, such that when the sensor array 10 is connectedto the printed circuit board 30, there is a one-to-one correspondencebetween the board contact pads 32 and the sensor contact pads 14.Tolerances in the positioning of the pads and trails of 0.001-inches canbe easily attained with modern manufacturing techniques, permittingprecise matching of the sensor and board contact pad patterns. Thesensor contact pads 14 and the board contact pads 32, via leads 33 andconnectors 34 that are disposed on the board 30, are the primary contactpoints through which the sensor array 10 connects with a flexibleelectronic platform, such as a computer and/or electronic testcircuitry.

[0049] To connect the sensor array 10 to the printed circuit boardcontact pads 32, a plurality of Z-axis connectors 40 can be used, asshown in FIG. 4. The Z-axis connectors 40 create the electricalconnection between the sensor contact pads 14 and the board contact pads32. In the embodiment shown in FIG. 4, the Z-axis connectors are formedfrom rubber or other elastomeric strips containing conductive metalparticles or wires for carrying current. These elastomeric conductorsare preferably designed to conduct electricity in only one direction toprevent cross-talk between closely spaced contacts. Other possibleconnectors that can be used to couple the sensor contact pads with theboard contact pads include cantilever or stick probes or other types ofspringloaded contacts, conducting adhesives, glues or epoxies, wirebonding, soldering, or direct contact between the sensor array and boardcontact pads 14, 32. Regardless of the specific type of structure, theZ-axis connectors 40 must create a reliable connection between thesensor contact pads 14 and the board contact pads 32, even when veryclosely spaced together, to ensure reliable coupling between the sensorarray electronic platform without cross-talk between adjacent contactpads.

[0050] The Z-axis connectors 20 can be placed in a frame or positioningfixture 42 that may be attached to the circuit board 30, as shown inFIG. 4. This allows the sensor and board contact pads 14, 32 to be linedup with each other precisely and coupled through the Z-axis connectors40 in a one-to-one relationship. The positioning fixture used withelastomeric connectors in the example experiments discussed below had asquare cavity, 2.002-inch+/−0.001″ tolerance, for precisely positioningof the sensor array 10, slots 41 to hold the connectors 40, and holes 43for optical/atmospheric access. The connectors 40 in the exampleexperiments discussed below were elastomeric connectors, such asFujipoly “Zebra Silver” connectors, having dimensions of 1 mm wide, 2″long, and 5 mm high. A compression plate 44 can be used to provideadditional security in the connection between the sensor and boardcontact pads 14,32, especially if the sensor array 10 and the printedcircuit board 30 are not bonded together. The compression plate 44 issimply placed on top of the sensor array 10, secured in place withscrews or other fasteners 46 and tightened until the sensor array 10,the Z-axis connectors 40 and the printed circuit board 30 are pressedfirmly together. The compression plate 44 may have a plurality of holes48 having the same configuration as the sensors 12 in the sensor array10 to allow optical testing of the sensor array 10, either alone or inconjunction with the electrical characterization according to thepresent invention, if desired, and permit gas exchange or evacuation.Holes 49 may also be provided in the printed circuit board 30 for thesame purposes.

[0051] The printed circuit board 30 may provide the primary electroniclink between the sensors 12 and any peripheral devices used to controland monitor the sensor array, such as the components in the flexibleelectronic platform. The printed circuit board 30 also can, in manycases, be considered part of the signal routing equipment (as opposed tobeing considered a part of the sensor array 10). In the embodiment shownin FIG. 3A and 3B, as noted above, the printed circuit board 30 has aplurality of connectors 34 arranged around the board's 30 periphery,leaving enough space in the center area of the printed circuit board 30for positioning the sensor array 10. The connectors 34 on the printedcircuit board 30 are preferably standard multiple-pin connectors so thata commercially available ribbon cable wire assembly can route signals toand from the sensor array 10 or couple the printed circuit board 30 withperipheral devices, such as a multiplexer and flexible electronicplatform. The illustrated structure, which was used in the examplesdiscussed below, is a Robinson-Nugent P50E-100STG 100-pin connector thatis compatible with the inputs of the multiplexer, but any othermultiple-pin connector can be used without departing from the spirit andscope of the invention. Each board contact pad 32 has an associated lead33 that extends from the board contact pad 32 to a pin on the connector34. It should be noted that the connection between the printed circuitboard 30 and the electronic platform need not be a physical connection,such as a ribbon cable, but can also be any type of wireless connectionas well as long as signals can be transmitted between the sensor array10 and the electronic platform.

[0052] A multiplexer may be included in the apparatus as signal routingequipment linking the sensor array 10 and the electric platform or testequipment being used, which is schematically represented in FIG. 6A. Themultiplexer used in the experimental examples discussed below was anAscor model 4005 VXI multiplexer module, containing four custom Ascorswitch modules (model 4517). Each switch module contains 64 2-wirerelays, in eight groups of eight relays per group, for a total of 128input connections per module (512 connections total, corresponding tothe number of contact pads on the sensor array). This design was chosenbecause it was easy to integrate with the embodiment having an 8×8 arraywith 8 contact pads per sensor. Thus, obviously, different designs maybe used without departing from the invention. Each switch module alsohas four output connections, which can be connected to different inputconnections by closing selected relays under computer control. Thesignal routing equipment shown in FIG. 6A emphasizes simultaneouscontact and connection of all of the sensors 12 to multiplexer inputs,with sensor selection being conducted by closing selected switches inthe multiplexer.

[0053] A preferred embodiment facilitates attachment of standardelectronic test and measurement equipment to the outputs of the signalrouting equipment. For the experimental examples discussed below, therewere eight terminals, one for each contact pad 14 on the sensor 12. Theoutputs were routed to a panel containing standard panelmounted BNCcoaxial connectors. However, again this design can be modified by thoseof skill in the art without departing from the invention. Generally, agiven pair of signals (e.g. AH and AL) can either be connected to acenter conductor and shield a single BNC terminal, which is electricallyisolated from the mounting panel, or be connected to the centerconductors of two separate BNC terminals whose outer shields areconnected to the system ground. This permits either single-ended or truedifferential connections to the sensors, with the connection mode chosenmanually for each pair of leads (e.g. A, B, C, and D) by means of atoggle switch. Thus, when a single sensor 12 is selected, the eightcontact pads 14 of the selected sensor 12 can be easily accessed fromthe panel of BNC connectors, using virtually any desired piece ofelectronic test and measurement equipment. Other types of terminals canof course be used.

[0054] For the apparatus used in the examples, thirty-two analogbackplane connections were provided between multiple 4517 switch modulesin the common 4005 multiplexer module, permitting highly flexibleconfiguration of the multiplexer. In addition to permitting selection ofone sensor at a time, the backplane connections permit selection of onesensor from each row at a time, with the outputs being made available onone or more 32-terminal output modules, which are also housed in the4005 multiplexer module and are connected to the analog backplane. Theflexible multiplexer design also permits the multiplexer to be used witharrays larger than 8×8 by permitting additional switch modules to beinserted into the housing and connected to the common backplane.

[0055] Again for the example experiments, the Ascor 4005 multiplexermodule was housed in a Hewlett-Packard model HP E8400A 13-slot VXImainframe. Communication with a computer was through a NationalInstruments GPIB-VXI/C interface module, which allows control of the VXIsystem via the computer's GPIB interface. The multiplexer was controlledfrom the computer by sending appropriate commands to the GPIB-VXI/Cinterface module. The software for controlling the multiplexerpreferably permits operation in two different modes. In both cases, agraphic representation of the sensor array 10 was shown on the computerscreen in the form of an array of “buttons.” In manual operation mode,the user selects one or more sensors by clicking on the correspondingbuttons and then instructing the computer to close the appropriateswitches. All eight connections to the selected sensor or sensors 12 arethen closed, while any previously closed connections on non-selectedsensors 12 are opened.

[0056] In automatic or scan operation mode, using the control softwarethe user again selects a group of sensors by clicking on thecorresponding buttons. The computer then closes the switches to thefirst sensor, performs a measurement procedure, and opens the switchesto the first sensor. The procedure is repeated for all of the sensorsselected by the user, scanning across each row from left to right andmoving from the top row to the bottom. Relays are not closed andmeasurements are not performed on unselected sensors. This software canbe changed to accommodate preferred modes of operation, includingrunning in parallel.

[0057] Once a sensor 12 is routed to the multiplexer output, manydifferent commercially available electronics components may be connectedto the sensor array 10 to input and output signals to and from thesensor 12. For example, if the sensor array is designed to measureresistance, a resistance meter that has one input is connected to themultiplexer output and can measure the resistance of any of the sensors12 that are connected to the multiplexer inputs. The multiplexer allowsa user to select any one of the sensors 12 on the array 10 and outputdata related to the resistance properties of the sample materialcontaining the selected sensor 12.

[0058] Alternative signal routing equipment is illustrated in FIG. 6B. Aprobe assembly 61 having probes 63 disposed thereon in an arrangementthat matches the sensor contact pad arrangement 14 on one or moresensors is position over a selected sensor 12 via a three-axistranslation stage. The three axis translation stage is preferablycontrolled by motors under computer control. The probe assembly 61itself may be moved, or the substrate 16 may be moved to position theassembly 61 and the substrate 16 relative to each other. To select asensor 12, the probe assembly 61 is positioned over the selected sensor12 and moved toward the substrate 16 to make electrical contact with theselected sensor's 12 contact pads. Wiring from the probe assemblyconnects the selected sensor or sensors to the electronic platform. Thespecific technology used for positioning the probe assembly 61 can beany positioning mechanism known in the art. The advantage of the sensorselection and signal routing system shown in FIG. 6B is that it largelyreduces or eliminates the need for a circuit board, multiwire cables,and multiplexer.

[0059]FIG. 5 illustrates one possible configuration for a genericflexible electronic platform that can be used in conjunction with thesensor array of the present invention. In this example, the outputs fromthe signal routing means, such as the multiplexer, are connected to amatrix switch 50 that is controlled by a computer 52. The matrix switch50 has a plurality of electronic test measurement instruments 54 thatcan be coupled to any or all of the multiplexer outputs. A user canselect which instruments to connect to particular sensors 12 in thesensor array 10 by either inputting instructions into the computer 52 toopen and/or close the matrix switch 50 connections by opening andclosing the connections manually, including manually rerouting cablesthat attach outputs to electronic inputs. Thus, this particular type offlexible electronic platform can output and read many different signalsrequired for measuring many different material properties with differentsensors, simply by changing the connections within the matrix switch 50.

[0060] Because the sensors 12 can be accessed using off-board circuitry,the inventive structure allows great flexibility in the manner in whichthe sensors are addressed. If a multiplexer is not used to controlsensor addressing, and if a separate electronics channel is provided foreach sensor as the signal routing means, then all of the sensors 12 inthe array 10 can be monitored simultaneously, allowing rapid parallelcharacterization of entire material libraries. If a multiplexer is used,any channel from any sensor 12 can be made available for input or outputvia its corresponding multiplexer terminal, and simultaneous butseparate of addressing individual sensors in different rows in the array10 is also possible. As illustrated in FIG. 6A, the computer 52 can alsobe programmed to conduct rapid serial measurement (addressing one sensor12 at a time), addressing all sensors 12 in a selected groupsimultaneously (such as heating each row to a different temperature tostudy thermal processing conditions), and simultaneously addressing onesensor 12 from each row (combined serial/parallel sensor measurement).All of these sensor accessing schemes can be implemented electronically,through software instructions to the multiplexer, without physicallyreconfiguring or rewiring any part of the apparatus because of theapparatus' modular construction, the interconnection structure, and theflexible electronic platform. Of course, if desired, any or all of thecomponents (e.g., the multiplexer, the printed circuit board 30, thesensor array 10, and the electronic test circuitry from the electronicplatform) can also be integrated in various ways to construct a morecustomized materials characterization unit.

[0061] An alternative structure for the sensor array 10 is shown inFIGS. 2C and 7. In certain applications, such as characterization ofliquid materials, it is not desirable to have the contacts between thesensor array 10 and the printed circuit board 30 located in the samevicinity as the sensors 12 themselves. The liquid materials would tendto contaminate the contact pads 14, 32, reducing the integrity of theinterconnection between the sensor array 10 and the printed circuitboard 30 and preventing reuse of the interconnection hardware, such asthe Z-axis connectors 40. To overcome this problem, the sensor array 10shown in FIGS. 2C and 7 directs the leads from all of the sensors 12 tothe edge of the substrate 16, away from the actual sensor sites. Contactbetween the sensor array 10 and the printed circuit board 30 is made atthe edge of the substrate 16, either with Z-axis connectors 40 as in thesensor array described above or with probe cards or probe arrays 70, asshown in FIG. 7. Cantilever probes 72 on the probe array 70 provide theelectrical link between the sensor array 10 and, for example, themultiplexer, the flexible electronic platform, or some other peripheraldevice.

[0062] Because the sensors 12 in the sensor arrays 10 shown in FIGS. 2Cand 7 are relatively flat and have their top surfaces physicallyexposed, a rubber gasket (not shown) containing holes in the samelocations as the sensors can be placed on top of the sensor array 10 tohold liquids in place over the sensors 10. The gasket can be pressed orbonded to the plate while the traces connecting the sensor array 10 tothe printed circuit board 30 can still be run along the substrate 16 toits edge. Further, because there is a clear optical path to the sensors12 from an overhead vantage point, the sensor array 10 can be used inconjunction with a camera or other optical sensing device, allowing evenmore material properties to be measured simultaneously. For example, ifthe sensors 12 in the array 10 are designed to measure the progress of acuring process via measurement of material dielectric constants, using acamera in conjunction with the materials characterization device of thisinvention allows detection and measurement of exothermic propertiesand/or temperature changes at the same time as measurement of thedielectric constant, further increasing the number of characteristicsthat can be measured at one time. See WO 98/15805, incorporated hereinby reference, for a discussion of optical screening techniques.

[0063] An alternative structure for the present invention is shown inFIG. 8. In this embodiment, the individual sensors 12 are cut apart andmounted onto individual sensor plates 80 to form “dipsticks” 82 thatpreferably extend vertically from the substrate 16. The spacing andformat of the dipsticks 82 may follow a conventional combinatorialchemistry format, such as an 8×12 array with 9 mm spacing, so that allof the dipsticks 82 in the array 10 can be dipped into standardcombinatorial chemistry wells 84 simultaneously, as shown in FIG. 8. Ina preferred embodiment, the wells 84 contain solutions comprising thematerials to be characterized dissolved in a solvent. Once the dipsticks82 are dipped into the wells 84 and removed, the solvent is allowed toevaporate and coat the sensors 12 with the sample material. Input andoutput signals are then sent to and from the sensors 12 in the same wayas described above to characterize the material properties. The liquidsin the wells can also be directly characterized as the while the sensorsare immersed in the wells.

[0064] Because the materials characterization system of the presentinvention has a modular, flexible structure, many different materialproperties can be monitored simply by changing the sensor structures inthe sensor array 10 and attaching different electronic components to thearray outputs or signal router outputs as needed, depending on thespecific material property to be measured. Thus, the sameinterconnection method and signal routing equipment may be used for alltypes of measurements, where the only components that need to be changedare the sensor array 10 itself (“plug-and-play” operation) and possiblysome specific electronic test circuitry in the electronic platform. Thisis much less expensive than purchasing a separate machine for measuringeach property. Also, as can be seen below, the sensor arrays 10themselves may be reusable in certain applications, reducingexpenditures for testing even further. The measurements obtained fromthe sensors 12 in the sensor array 10 of the present invention can bedirectly correlated to known testing results. In other words, theresults obtained from the sensor arrays 10 correlate with resultsobtained from conventional materials characterization methods. Thisadvantage of the present invention will be highlighted in greater detailwith respect to the experimental examples described below.

Thermal Analysis Background

[0065] Thermal analysis is one of the most generally useful techniquesof materials analysis, particularly measurements of heat capacity. Inmany cases for thermal analysis, it is important that the sample beinganalyzed is thermally isolated from its environment to a large degree.Thermal isolation insures that heat flows into and out of the sample andthe associated changes in the sample temperature may be accuratelydetermined and are not masked by much larger heat flows associated withother objects, such as the sample holder or substrate, heater andthermometer, etc. Samples produced in combinatorial materials synthesismay consist of films, created by physical vapor deposition techniques(evaporation, sputtering, etc.) or by deposition of a liquid solution orsuspension and subsequent evaporation of the solvent. The samplespreferably have small lateral dimensions (e.g. 1 mm or less), to allowmore samples to be deposited on a given area. A sensor designed forthermal analysis of combinatorial libraries must therefore allowaccurate measurements to be made on very small samples that are packedclosely together on a substrate. Although thermal isolation of minutesamples initially poses a challenge, it also offers an advantage in thatthe thermal time constants for internal equilibration of the sample,heater, and thermometer are greatly reduced, permitting more rapidmeasurements to be made.

[0066] Thermal isolation of small-area thin film samples may be mosteasily achieved by using a thin film of low thermal conductivitymaterial to support the sample, where the support's thickness iscomparable to or less than that of the sample. The heat capacity andthermal conductance of the support are thus comparable to that of thesample film, can be independently measured, and can be subtracted frommeasurements made with a sample present. Further isolation during themeasurement can be achieved by use of various modulated or pulsed heatcapacity measurement methods, which will be discussed below.

[0067] Issues affecting the design of a thin film calorimeter are thematerials used for fabricating the substrate and thin support membrane,the materials used for fabricating the heater and thermometer, thegeometry of the heater and thermometer, membrane, sample, and substrateas they affect the temperature profile and transport of heat, and theway in which the sensors can be connected to an interface so that usefulinformation can be obtained from the sensors. FIGS. 9A through 9Cillustrate preferred sensor structures for use in thermal analysisapplications with thin film samples. Although the figures illustrate thestructure of one individual sensor, it is understood that all or part ofthe sensors in the sensor array can be manufactured on the substrate 16simultaneously.

[0068]FIG. 9A is a preferred thin-film structure for conducting thermalanalysis of a thin-film sample 90. A micro-thin membrane 94, whichsupports the sample material 90, is preferably made of silicon nitride(Si₃N₄) on a silicon wafer substrate 92. The substrate 92 preferably hasa plurality of membranes 94 that are formed thereon in the desiredsensor array arrangement. To form the membranes 94, a thin film ofsilicon nitride 95 is deposited on the top and the bottom surfaces ofthe silicon wafer 92. The thickness of the silicon nitride film 95 ispreferably between 500 angstroms and 2 microns, as this thickness can beeasily produced by chemical vapor deposition and other techniques; italso corresponds to typical thickness' of the thin film samples 90 to bestudied. The desired membrane pattern is then created on the bottomsurface of the silicon wafer 92 to open up “windows” 96 in the siliconnitride film 95, exposing the silicon 92 at selected locations. Theentire wafer structure is dipped into an etching solution, such aspotassium hydroxide, to erode or etch away the silicon 92 exposedthrough the windows and form the structure shown in FIG. 9A. Regardlessof the specific etchant used, it should etch silicon but not etchsilicon nitride. Because of silicon's crystal structure, the etchingprocess forms a well 97 with sloping walls through the silicon layer 92and which stops at the top silicon nitride layer 95. The resultingstructure is a suspended, micro-thin silicon nitride membrane 94supported by silicon 92. The well 97 makes the sensor array structureparticularly suited for depositing films from solids dissolved in asolvent, as a drop of the solvent can be held in the well 97 duringdrying. The well 97 can also contain liquids which are being tested.

[0069]FIGS. 9B and 9C illustrate one possible heater/thermometer pattern100 that can be printed on the membrane 94 to form a complete thermalanalysis sensor. As can be seen in the figure, the preferredheater/thermometer pattern 100 is designed so that the thermometerportion 102 is much smaller than the heater portion 104 and so that thethermometer 102 is located in the center of and surrounded by the heater104. The heater 104 is still sufficiently small so that the edges of theheater/thermometer pattern 100 are isolated from the edges of themembrane 94. These design features give the sensor 12 several desirableproperties that make it useful for conducting rapid heat capacitymeasurements on thin film samples. The time constant for equilibrationof the heater 104 with the thick part of the substrate 92 (beyond theedge of the “window”) is much longer (slower) than the time constant forinternal equilibration of the portion of sample 90 adjacent to theheater 104 and thermometer 102, since the time constant is proportionalto the square of the distance over which the heat must diffuse. Thetemperature profile across the heater 104 may to some extent have anon-uniform dome-shaped profile due to heat flow from the center of theheater 104 outwards; placing a small thermometer 102 in the center ofthe heater 104 allows measurement of the temperature in a region whosetemperature is much more uniform than the temperature of the entireheater 104.

[0070] The heater/thermometer 100 is preferably printed on the flat sideof the membrane 94 via lithography so that the sample 90 can bedeposited on the membrane 94 within the “well” portion 97 and becharacterized without actually touching the heater/thermometer pattern100. The membrane 94 prevents direct physical contact between theheater/thermometer 100 and the sample, yet is still thin enough tocreate intimate thermal contact between the heater/thermometer 100 andthe sample 90 and allow heat to conduct through the membrane 95 to warmthe sample 90 and measure its thermal characteristics. This feature isparticularly useful when characterizing metals, where direct physicalcontact between the heater/thermometer 100 and the sample 90 wouldcreate a short circuit in the heater 104. Heater/thermometer leads 106are connected to the sensor contact pads 14 or are otherwise configuredfor coupling with the flexible electrical platform so that the powerinput and the sample's temperature can be monitored and controlledelectrically. Thus, it can be seen that in some embodiments, coatingsmay be used on the sensors to protect the sensors from the samples orvice versa.

[0071] The specific configuration for the micro-calorimeter array andthe system architecture for coupling the sensors 12 in the array 10 tothe electrical platform can be any structure desired by the user as longas electrical signals can be sent to and read from each individualsensor 12 in the array 10. The micro-calorimeter used in the exampleexperiments was custom manufactured so that the substrate 92 was a 0.5mm silicon wafer, with 0.5 μm of low-stress LPCVD silicon nitridedeposited on both sides. The silicon nitride membranes 94 were 2 mmsquares and prepared by known procedures. To produce the metallizationpatterns, a liftoff procedure was used. Photoresist is spun on to thefront side of the wafer, and is patterned by photolithography using astepper. 50 Å of Ti was then deposited on the photoresist and theexposed portions of the substrate, followed by 2000 Å of Pt. The Tilayer was added for adhesion purposes. The photoresist was thendissolved, leaving metal in the desired pattern on the substrate. In theembodiment of the heat capacity sensor used in the examples, the heater104 consists of a serpentine pattern, with 60 μm lines separated by 20μm spaces. The thermometer 102 is a smaller area serpentine pattern,with 20 μm lines and 20 μm spaces. Following liftoff, the wafer was cutinto the form of a square measuring 2.000″+/−0.001″ precision, using adicing saw. Accurate dicing of the wafer is needed for accuratepositioning of the wafer relative to the circuit board, using thepositioning frame. A dimension of 2″ was chosen to allow the substratesto be inserted into combinatorial vapor deposition equipment.

[0072] An alternate material for the substrate is a polymer sheet. Aparticularly suitable polymer is a material called Kapton®, which ismanufactured by DuPont. Kapton is thermally stable and can withstandtemperatures up to 350-400 degrees C. without deterioration. Kapton isoften sold in sheets ranging from 6 microns thick to 100 microns thick,and the heater/thermometer design, such as that shown in FIGS. 9B and9C, can be printed directly onto the film via lithography or othertechniques. To suspend the Kapton® sheet when conducting thermalanalysis, the contacts can be printed such that they are all at theedges of the sheet, as shown in FIGS. 2C and 7, and the sheet can bestretched and clamped at the edges to connect the contacts on the sheetwith corresponding contacts associated with the flexible electronicplatform. If the samples are not electrically conducting, then theentire side of the sheet opposite the side containing the sensors can becovered with a layer of metal, which can be used as a blanket heater forheating all of the samples simultaneously, either via a DC signal or amodulated signal. As noted above, the inventive structure providesenough flexibility so that selected samples can be heated individually,simultaneously, or in any grouped combination simply by changing theelectronic signals sent by the electronic platform.

[0073] In applications where larger amounts of material are availablefor analysis, e.g. samples which may be 10's to 100's of micrometersthick, it may be possible to use thicker substrates. In these cases,thermal isolation can be improved by using substrates with low thermalconductivity, such as glass, or by micromachining a gap or “moat” 110around a sample support 112, which is spanned only by microbridges 114of material, as illustrated in FIG. 10. The heater/thermometer patternmay be printed on the sample support 112. The microbridges 114 hold thesample support 112 in place on the substrate while minimizing heatleakage, and also act as supports for wires which must pass into and outof the sensor for coupling with the electronic platform.

Experimental Example: Thermal Analysis of Polymers and Metal Alloys

[0074] The modular sensor array structure described above isparticularly useful for rapidly measuring thermal properties ofcombinatorially synthesized libraries of polymers. The predominant useof heat capacity measurements with polymers is for the determination ofthe temperatures at which phase transitions occur and the identificationof the types of phase transitions occurring (generally either glasstransitions or melting points). This information can then be used in twogeneral ways.

[0075] In some specific cases, it is desirable to have a phasetransition occur at a particular temperature, and the goal ofcombinatorial synthesis might in part be to tune the polymer's physicalproperties until that value is achieved. For example, the glasstransition temperature is an important parameter for the polymerparticles used in latex paints, as it strongly affects how the latexparticles will coalesce and form a film under given environmentalconditions. A latex for a given coating application may have to fulfillmany other conditions as well, related to properties such as adhesionand weatherability.

[0076] It is a common practice to try to achieve several desirableproperties simultaneously by making random copolymers, containing anessentially random sequence made from two or more different monomers.The types and numbers of monomers used and their relative proportionscan be varied in many different combinations, to attempt creating apolymer that simultaneously fulfills all of the desired criteria.However, adding a monomer that improves adhesion may reduce the glasstransition to an unacceptable value, for example. Thus, being able torapidly measure the glass transition temperature (in addition to otherproperties) for many hundreds of random copolymers allows the balancingof different physical properties to be done much more rapidly.

[0077] An example is shown in FIG. 1A, where the glass transitions havebeen determined for a series of styrene-co-butyl acrylate randomcopolymers with different styrene contents, using the specific detailsdiscussed above. The random copolymers were synthesized by Atom TransferRadical Polymerization (ATRP) at 140 C. for 15 hours, using CuCl withtwo equivalence of 4,4′-dinonyl-2,2′-bipyridine (dNbpy) as the controlagent and (1-chloro)ethyl benzene (PhEtCl) as the initiator. Themonomers styrene (S) and n-butyl acrylate (BA) were combined to make 11solutions ranging from 100% S to 100% BA in steps of 10 volume %. Acatalyst stock solutions was made in toluene by combining 1 part PhEtClwith 1 part CuCl and 2 parts dNbpy. For each of the 11 monomer stocksolutions were set up five polymerizations with varying ratios ofmonomer to initiator, by varying the amount of catalyst stock solutionadded. This led to a 55 element array of random co-polymerizations thatvaried in the x-axis by the composition of monomers, and in the y-axisby the theoretical molecular weight (ranging from 10,000 to 50,000).

[0078] The samples in the example were chosen from the styrene-richportion of the library, in order to produce Tg's above room temperature.The example of the inventive apparatus and method described here doesnot contain a means for cooling samples below room temperature; however,as is obvious to anyone skilled in the art, this can be accomplishedeasily in many different ways. The molecular weight of the polymers usedwas approximately 30,000 gm/mol. The polymers were dissolved in tolueneat room temperature to a concentration of approximately 2%. Small drops(approximately 5 μl) of the solutions were manually pipetted onto thesensors, and allowed to dry in air until a film was formed. The heatcapacity data shown in FIG. 11A were obtained using the inventiveapparatus and method, following the “3ω” measurement procedure, which isdiscussed below.

[0079] The glass transitions of the polymers can clearly be observed asa “step” in the heat capacity vs. temperature data. This is identical tothe type of behavior observed using traditional differential scanningcalorimetry to measure the heat capacity, and the data are of entirelycomparable quality with respect to the sharp definition of the featureassociated with the glass transition. The glass transition ofpolystyrene occurs near 100° C., in fair agreement with known results.It should be noted that these data were taken using an approximatecalibration for the temperature sensor; improved calibration procedureswill naturally yield more quantitatively precise values for Tg.

[0080] In addition, the glass transition temperature Tg can be seen toclearly decrease to lower temperatures with increasing butyl acrylateincorporation. This is entirely in accord with the known behavior ofrandom copolymers, which typically show a glass transition temperatureat a value intermediate between that of the pure component polymers (a100% butyl acrylate polymer would have a glass transition temperature Tgof approximately −75° C.). However, the total time required to acquirethis data using the inventive method is less than 2 minutes. Similarmeasurements using a conventional differential scanning calorimeterwould take several hours or more.

[0081] Another example in which the precise value of a phase transitiontemperature is important is in the area of thermally responsivepolymers, including polymers with crystalline side chains, liquidcrystallinity, etc. Thermally responsive polymers are important to awide variety of applications. Thermal measurements on polymers are alsoimportant in determination of a polymer's performance under differentenvironments, including solvent, vapors, humidity, radiation, oxidationand the like. For example, the sample polymers may be tested afterexposure to a certain environment or may be tested while being exposedto the environment.

[0082] Even more generally, however, information about phase transitionscan give a great deal of insight into the chemical and physicalstructure of the polymer being studied, which in turn can be relatedeither to the success or failure of a particular synthetic strategy, orto the suitability of the material for applications involving propertiesother than the melting or glass transition temperatures. Thus, thermalanalysis is extremely useful within a combinatorial polymer synthesisprogram, as it allows a scientist to rapidly assess variations inpolymer physical properties due to different catalysts, processconditions, etc., as well as to assess whether or not a polymer with adesired chemical composition or architecture has in fact beensynthesized. The following examples will illustrate these points.

[0083] Even in the case of polymers made from a single monomer (e.g.ethylene), the physical properties of the polymer will vary tremendouslydepending on the architecture of the polymer, e.g. the molecular weight,and the degree and type of branching. For example, high densitypolyethylene (HDPE) and paraffin (wax) are chemically similar, butdiffer in their molecular weights and the amount of bridging betweencrystallites. The greater number of chain ends in paraffin severelydisrupts the crystalline packing of the chains, in comparison to HDPE,leading to vastly inferior mechanical properties. The difference inphysical properties is also directly manifested in a lower melting pointfor paraffin in comparison to HDPE.

[0084] Other factors which result in a reduced melting point arebranching, and comonomer incorporation. Branching not only reduces thevalue of the melting point, but also reduces the total degree ofcrystallinity. Crystalline polymers in fact consist of both crystallinedomains, or crystallites, and amorphous regions between the crystallitesdue to chain folding and chain ends. Generally, the greater degree ofbranching, the larger the amorphous fraction of the polymer. Theamorphous regions display a glass transition, and by measuring the heatcapacity signals associated with both the glass transition and themelting point, one can obtain information on the degree of crystallinityof the polymer, which in turn strongly affects the mechanical propertiesof the polymer. Similar considerations apply for polymers whichincorporate comonomers.

[0085] Thus, in evaluating combinatorial libraries of ethylenecatalysts, a rapid determination of the melting point and degree ofcrystallinity can give a good qualitative picture of what type ofpolyethylene is being produced by the catalyst. This adds a great dealof information to lower level screens such as the degree of catalystactivity and the polymer molecular weight, information which is moreclosely related to the end uses of the polymer produced by a givencatalyst.

[0086]FIG. 11B shows heat capacity curves created with the apparatus ofthis invention for a series of ethylene-co-methyl acrylate randomcopolymers. The polyethylene-co-methyl acrylate copolymers werepurchased from Aldrich, and the Aldrich catalog numbers, the percentagesof methyl acylate incorporation, and melting points according to themanufacturer were: #43076-5, 9% MA, MP=93° C.; #43264-4, 16% MA, MP=85°C.; and #43075-7, 29% MA, MP=48° C. The ethylene co-polymers weredissolved at a concentration of approximately 5 wt % intrichlorobenzene, a high boiling point solvent, at 150 degrees C.Approximately 5 to 10 ml were dispensed onto each sensor 12 and werekept in place by the naturally occurring wells beneath the siliconnitride membranes 94. The solvent was allowed to air dry, leaving apolymer film 90 deposited on each membrane 94. The heat capacity datawas obtained using the 2ω method, as described below. A broad peak inthe heat capacity is observed, marking the melting point. This peak isdue to the latent heat associated with melting of crystalline portionsof the polymer and the data are comparable to the results that may beobtained by traditional DSC. The reduction of the value of the meltingpoint and the degree of crystallinity with increasing methyl acrylateincorporation can be easily seen in FIG. 11B.

[0087] Heat capacity measurements can also be used to gain informationon the architecture and microstructure of glassy (entirelynon-crystalline) polymers. For example, a “random” copolymer of a givencomposition may be either random or “blocky”, depending on whether ornot the comonomers alternate in a random way or tend to occur in longer“runs” of a given monomer type. The degree of randomness or blockinesscan affect the end properties of the material. The degree of blockinesscan be assessed through heat capacity measurements: a random copolymertends to have a single broad glass transition, at a temperatureintermediate between the Tg's of the constituent monomers. If the randomcopolymer is actually blocky, however, two distinct Tg's may beobserved, corresponding to domains which form almost entirely from longruns of one or the other monomer.

[0088] In a similar manner, heat capacity measurements can distinguishbetween immiscible and miscible polymer blends or betweenphase-separated or phase-mixed block copolymers. Phase-mixed systemsshow a single Tg, while phase separated systems show two distinct Tg's.Even in the case of a phase separated blend, small amounts ofmiscibility will occur, i.e., the two phases are not “pure”. This canalso be assessed using Tg measurements, as the two Tg's will be somewhatshifted from the values for the pure polymers.

[0089] The above examples illustrate the many ways in which thermalanalysis data can be used to gain important information on the structureand physical properties of polymers. This information can be used toevaluate the success or failure of a particular synthetic route inmaking a polymer with a given chemical composition and physicalstructure/architecture; or to judge the suitability of a particularpolymer for a given application. Within the context of a combinatorialmaterials science approach to developing new polymer syntheticstrategies or new polymeric materials, in which many catalysts, processconditions, chain compositions and architectures, etc., will beattempted, it is highly desirable to be able to obtain thermal analysisinformation in a rapid fashion.

[0090] The sensor array method and apparatus of the present inventionhas a significant advantage over other thermal analysis methods andapparatuses because it can characterize many different materialssimultaneously and quickly. Instead of obtaining only one heat capacityscan per unit time, the inventive method and structure can obtain tensor even hundreds of heat capacity plots in the same amount of time.Further, the sensor for this particular application obtains data thatcan be readily correlated with known data, e.g., from a conventionaldifferential scanning calorimeter (DSC), in that the heat capacity ofthe sample can be directly measured and plotted. Thus, the sensor outputneeds only minimal processing to generate data that can be easilyinterpreted.

[0091] The method for utilizing the present invention is also simple,and further facilitates the rapid analysis of numerous samples. Once alibrary of, for example, 100 polymers is created via combinatorialmethods, each polymer may be deposited on the microcalorimeter sensorarray of the present invention to measure each polymer's thermalproperties. To form a sample 90, a small amount of solution containingthe polymer sample is placed on each sensor 12 and allowed to dry,leaving a film of the polymer behind. This can be done one sample at atime or multiple samples at a time manually or automatically, such as byusing a liquid dispensing robot with a single or multiple syringe tip.In a preferred embodiment, the sensor array 10 has a standardizedcombinatorial chemistry format so that the polymers may be depositedsimultaneously on multiple sensors 12 in the sensor array 10, usingknown combinatorial tools such as multiple syringe/multiple tippipettes, containing 4, 8, 12, or even 96 pipette tips possibly with thestandard 9 mm spacing.

[0092] Once the solvent has evaporated, leaving a polymer film sample 90on each sensor 12, the sensor array 10 is simply connected to theelectronic platform. This will be done, for example as shown in FIG. 4,by inserting the sensor array 10 in a positioning fixture 42 attached tothe printed circuit board 30 and applying pressure to the sensor arraysubstrate 16 by tightening screws 46 or other fasteners (such as clipsor clamps) on a compression fixture 44, insuring good contact betweenthe sensor array 10 and the printed circuit board 30. Preferably, theprinted circuit board 30 is housed in a chamber that can be evacuated.This eliminates heat losses to the atmosphere, and noise in thetemperature measurements due to convection. A heat capacity scan is thengenerated for each sensor 12 (typically in less than a minute),obtaining each material sample's crystallinity/amorphous properties,melting point, glass transition point, and other material characteristicinformation. The entire measurement procedure may be controlled andexecuted by a computer program in the electronic platform. Using thesoftware, the user initially specifies which samples in the array 10 areto be analyzed and provides other measurement information, such as thetemperature sweep rate and modulation frequency.

[0093] As a result, in this example, the heat capacity plots can beobtained for 100 samples in about 90 minutes or less, compared to aroundone or two samples in 90 minutes for known materials characterizationdevices, such as standard differential scanning calorimeters. Bycomparing and analyzing the heat capacity plots of each material in thelibrary quickly, a user can select which polymers in the library havethe most desirable physical properties for a selected application ordetermine whether or not a given synthetic strategy and set of startingingredients has in fact produced a polymer of a desired architecture andassociated physical properties.

[0094] Of course, thermal analysis is not limited to polymers. The sametype of analysis can also be used to characterize inorganic solid statematerials, such as glasses, metal alloys, and compounds.

[0095]FIG. 11C shows an example using this invention of a glasstransition (Tg) measurement in a low-Tg (400° C.) silica glassmanufactured by Ferro corporation, type 7578 crystallizing solder glass.Such “solder glasses” are widely used as sealing or fusing materials ina variety of specialized electronics and other applications, and theability to rapidly measure Tg of different combinatorially synthesizedsilica glass formulations would be highly desirable in the developmentof new specialty glasses. The glass used in this example has a glasstransition temperature at approximately 395° C. according to themanufacturer. The glass is normally obtained in powder form, and thepowder was formed into a disk for this experiment by placing the powderin a mold and sintering at 450 degrees C. for four hours. A 1 μm thickfilm was then deposited onto the sensor array using laser ablation. Themeasurements were made using the 3ω method, described below.

[0096] The present invention is also useful in the context of a searchfor new bulk amorphous metallic alloys, metals which do not have aregular crystal structure, and which display a reversible glasstransition much in the same manner as silica glasses. Such materials arehighly desirable for their unique high strength and resiliency incomparison to conventional alloys. Amorphous alloys typically consist ofthree or more different metal atoms, and achieving desirable physicalproperties such as strong glass forming ability and a low Tg requiressynthesizing many different alloys with slight variations incomposition. Thermal analysis is a widely used technique for analyzingthe glass transition and other phase transitions in candidate amorphousalloy materials. Thus, the combination of combinatorial synthesis andrapid thermal analysis is a powerful technique which can be used in thesearch for new amorphous alloys.

[0097]FIGS. 11D and 11F show examples of the determination of meltingpoints of several pure metals, and FIG. 11E shows a thermal analysisscan for a compound using the apparatus and method of this invention.The aluminum and lead films, each about 0.5 μm thick, shown in FIGS. 11Dand 11F, respectively, were deposited on the sensors by RF sputtering,using single element sputtering targets. The Al₃Mg₂ film was depositedas a multilayer film, using a combinatorial sputtering chamber. The filmas deposited contained alternating layers of 24 Å of AL and 26 Å of Mg.This layering was repeated 65 times, for a total film thickness of 3250Å. The layers mix to form the desired compound during the initialheating stage, which is below the melting point. The results shown inFIGS. 11D through 11F were obtained using the 3ωmethod, as described inthis application. Thus, the present invention, when combined withcombinatorial synthesis of thin solid films, can be used to map out theoutlines of entire binary, ternary, and higher order phase diagrams.This can be extremely useful in the search for new solid statecompounds, alloys, and other materials.

[0098] The method, which will be described in greater detail below, andthe apparatus of the present invention, which has been described, canthus analyze libraries of metal alloys, glasses, and other solid statecompounds and materials having varying compositions, to detect theoccurrence of important phase transitions. Again, because of the sensorarray 10 and library format used in the invention, a large number ofmaterials can be generated and screened in a short period of time. Inthe preferred method of deposition, the library of thin film materialsis directly produced on the sensor array substrate 16, usingcombinatorial masking and deposition techniques. See, e.g., WO 98/47613,incorporated herein by reference. Solid state films can also be producedfrom liquid precursors by sol-gel processes.

[0099] In short, material samples 90 are placed in intimate thermalcontact with the membrane 94 using vapor deposition techniques or bydissolving the sample in a solvent, depositing the solution on a sensor12 and allowing the solvent to dry to form a thin sample material filmon the membrane 94 of the sensor 12. The thinness of the membrane 94 andthe sample 90 allows the sample 90 to be heated through very quickly,making rapid scanning of the sample over a wide temperature rangepossible while still obtaining clear thermal characteristic plotsshowing phase transitions. This specific embodiment of the invention canscan over several hundred degrees and obtain heat capacity data for agiven sample in 10 to 30 seconds, compared with 30 minutes to 2 hoursfor conventional calorimeters. This processing speed is further enhancedby the invention's array format, allowing parallel or rapid serialscanning of multiple samples which are deposited on a single substrate,and increasing the number of samples tested per unit time to as high as64 or more samples in 15 minutes.

Experimental Example: Thermal Analysis with Temperature Modulation

[0100]FIGS. 12A through 12H and FIGS. 13A through 13F illustrate thermalanalysis using temperature modulation. The preferred sensor structurefor conducting this type of analysis is the structure described aboveand shown in FIGS. 9A through 9C, but other thermal sensor structurescan be used without departing from the spirit of the invention. Thefollowing discussion of non-modulated calorimetry will provide anexplanation of the theory behind heat capacity measurements and willillustrate why temperature modulated calorimetry is the preferred methodfor making heat capacity measurements with the sensors 12.

[0101] In an ideal or simplistic heat capacity measurement, all heatinput into a sample is retained by the sample, resulting in increases inthe temperature or change of the physical 15 state of the sample. Theheat capacity can then be determined as the ratio between the rate ofheat input and the rate of temperature increase, C_(p)=ΔQ/ΔT. Inreality, some of the heat input to the sample is continuously lost tothe environment through conduction, convection, radiation, etc. In orderfor the results of a heat capacity measurement to be meaningful, eithersome procedure must be implemented to measure or account for the heatenergy lost to the environment, as is done in differential scanningcalorimetry by means of an “empty cell” reference sample, or the rate atwhich heat is input to the sample must be much greater than the maximumrate at which heat is lost, so that losses may be neglected whilemaintaining a good approximation of the sample's heat capacity.

[0102] In the latter case, the entire measurement must be completed in atime shorter than the thermal relaxation time t1 of the sample, where t1is the time that it takes for the sample to come to equilibrium at a newtemperature when the heat input level is changed to a new value. If theheat input is set to zero, t1 is the time constant for the sample toreturn to the temperature of the environment. The relaxation time isgiven by t1=C_(p)/k, where C_(p) is the heat capacity and k is thethermal loss constant to the environment. The reason for conducting arapid (less than t1) measurement is easy to understand: if the power issuddenly turned up to a certain level, the temperature will initiallyincrease rapidly and the losses to the environment will be negligible,since the sample is initially at nearly the same temperature as theenvironment. However, after a time of approximately a few times t1, thesample's temperature saturates or plateaus to a limiting value, as theheat input and losses to the environment become exactly equal. Thus,heat losses to the environment can be neglected only if the temperatureincrease is conducted over a time much shorter than t1.

[0103] For small, microthin samples, such as those tested in theinvention, the time constant t1 can be quite short, typically 0.1seconds, due to the sample's very low heat capacity. The hightemperature ramp rates which must be used with such samples in a“continuous sweep” calorimetry experiment, e.g. 100's to 10,000's ofdegrees per second, make analysis of many phase transitions difficult orimpossible, particularly in more complex materials. If a much slowerramp rate is used, then an equilibrium prevails between the heat inputand the losses to the environment, and generally no information can begained about the heat capacity. Increasingly complex materials may takeincreasingly long times to complete structural rearrangements that occurat a phase transition, which involve collective motions andrearrangements of many atoms or molecules. Therefore, it is desirable touse a measurement method in which the heat capacity can be measuredwhile the average temperature is varying at an arbitrary rate.

[0104] AC calorimetry, when combined with the sensor design of theinvention, is a preferred way to obtain a rapid determination of heatcapacity versus temperature with a minimum of off-line data analysis,but without requiring prohibitively fast scanning of the averagetemperature. Although this discussion focuses on modulated calorimetry,other calorimetric methods may be used in conjunction with the sensorsor system of this invention, including methods based on measurements ofthe thermal relaxation time or methods in which the entire measurementis performed in a time that is shorter than the thermal relaxation time,which are well known in the art.

[0105] The general concepts of AC calorimetry will now be explained inconjunction with FIGS. 12A and 12B, which are general to the concepts.In AC calorimetry, the power input to the sample consists of a slowlyvarying average value P(t), and a modulated part ΔP. The heater powermodulation frequency 2ω (corresponding to modulation of the heatervoltage V_(H)(t) at a frequency ω, since P=V²/R) is chosen such that theperiod Δt=π/ω is much shorter than the time constant t1 forequilibration of the sample with the external environment, but muchlonger than the time constant t2 for internal equilibration between thesample, heater, and thermometer.

[0106] If the frequency ω is too low (ω<<π/t1), then the total powerinput is always equal to the losses to the environment; in this case,the temperature modulation is in phase with the power input modulation,contains information only about the thermal losses to the environment,and contains no information about the heat capacity. If the frequency ischosen so that ω>>π/t1, however, the sample temperature modulation lagsbehind the power input modulation by a phase angle of 90°, because thereis insufficient time during a cycle for the sample to reach thetemperatures corresponding to the maximum and minimum power inputs. Thelarger the heat capacity of the sample, the more slowly it responds tothe power modulation, and the smaller the resulting temperaturemodulation will be. Under these conditions, the temperature modulationamplitude ΔT is given by ΔT=ΔP/2ωC_(p), where ΔP is the amplitude of thepower modulation and C_(p) is the heat capacity. Thus, the heat capacityis inversely proportional to the temperature modulation amplitude, for afixed ΔP. If the frequency is too high, however, i.e. ω>>π/t2, then theheater, thermometer, and sample are not in equilibrium with each other,and the thermometer thus does not give accurate information about theresponse of the sample temperature to the heater power input.

[0107] Thus, AC calorimetry generally involves measuring both theaverage temperature and the temperature modulation amplitude for a givensample, with an appropriately chosen frequency 2π/t1<<ω<<2π/t2, as theaverage temperature is varied. The heat capacity is given by Cp=ΔP/2ωΔT.

[0108]FIGS. 12C through 12H are explanatory diagrams of a particularembodiment of a heat capacity measurement system and measurement method,which uses the preferred sensor design discussed above and the ACcalorimetry technique. This particular embodiment is referred tothroughout this specification as the “2ω method”. FIGS. 12C and 12D arerepresentative diagrams explaining the 2ω method, while FIGS. 12Ethrough 12H show examples of input and output signals according to thismethod.

[0109] The voltage signal to the heater, V_(H)(t), is the sum of aslowly varying average value V_(H,0)(t) and a modulationv_(H)(t)=v_(H)e^(iωt) at frequency ω, i.e.,V_(H)(t)=V_(H,0)(t)+v_(H)e^(iωt). The input power is V_(H)²(t)/R_(H)=[V_(H,0) ²(t)+2V_(H,0)(t)v_(H)e^(iωt)+v² _(H)e^(2iωt)]/R_(H),and contains modulations at frequencies of both ω and 2ω. Thetemperature of the sample is monitored by measuring the resistance ofthe thermometer 102, R_(TH)(t). In the 2ω method, this is done bypassing a small DC current I_(TH) through the thermometer 102 andmeasuring the voltage V_(TH)(t). For many metals, the resistance varieslinearly with temperature, and can be parameterized by the formulaR(T)=R(T=T₀)[1+α(T−T₀)], where α is a constant characteristic of themetal, and T₀ is an arbitrary reference temperature. Thus, thetemperature can be calculated directly from the thermometer voltage,using the formula T=T₀+[(V_(TH)/V_(TH)(T₀))−1]/α, if α, T_(0 and V)_(TH)(T₀) are known.

[0110] The average temperature and the temperature modulation atfrequencies ω or 2ω can easily be determined over the course of anexperiment by a number of means. The average temperature is most easilyobtained by passing the thermometer voltage signal through a low passfilter 120 with a suitable cutoff frequency, which removes themodulation, measuring the filtered thermometer voltage with ananalog-to-digital converter 122, and calculating the temperature usingthe formula given above. The modulation is most easily and accuratelymeasured using a lockin amplifier 124, with the reference frequency setat ω or 2ω depending on which frequency is being monitored. Othertechniques can also be used, such as an AC voltmeter with a narrow bandpass filter on the input, a spectrum analyzer, or direct recording ofthe waveform and subsequent off-line analysis by fast Fourier transform.

[0111] It is preferred to monitor and analyze the signal at frequency2ω. The principal reason is that the power modulation ΔP(2ω), given byv² _(H)/R_(H), varies relatively little during the experiment, varyingonly due to changes in the heater resistance R_(H) as the temperature isvaried. In contrast, the power modulation ΔP(ω)=2V_(H)(t)v_(H)/R_(H) iszero when the average heater voltage is zero, and varies over a muchwider range during the course of an experiment due to the lineardependence on V_(H)(t). This leads to vanishing sensitivity near thebase temperature, and a large variation in the signal-to-noise ratioover the course of an experiment.

[0112] The heat capacity is given by Cp=ΔP/2ωΔT, as described earlier.Because R_(H) increases with temperature, the input power modulationΔP=v_(H) ²/R_(H) decreases with increasing temperature. This leads to adecrease in the temperature modulation amplitude, independent of anychanges in the heat capacity. This must be accounted for in analyzingthe data. Because ΔP is inversely proportional to R_(H), the heatcapacity is proportional to 1/R_(H)ΔT, since v_(H) and ω are constantduring a given experiment. Although R_(H) can in principle be preciselydetermined by an additional measurement, e.g., by monitoring the DCcurrent drawn by the heater in response to the DC voltage V_(H)and anabsolute value of Cp determined, it is a reasonable approximation formany purposes to assume that the heater and thermometer are at the sametemperature, and substitute RTH (which is already being measured) forR_(H).

[0113] Thus, if one is only interested in identifying prominent featuresin the heat capacity curve that are associated with phase transitions orother significant thermal events, as is often the case, and is notinterested in the precise absolute value of the heat capacity, then theheat capacity can be approximated (up to a multiplicative constant orscaling factor) by 1/[<V_(TH)>ΔV_(TH)(2ω)], where the denominator is theproduct of the DC and modulated values of the thermometer voltage. Aplot of this quantity vs. the temperature (which is derived from V_(TH))captures all of the essential information in the heat capacity curve.More precise analysis methods may be used to obtain an absolute value ofthe heat capacity, without departing from the scope and spirit of theinvention.

[0114] It is now possible to explain more clearly why AC calorimetry, asembodied by the 2ω method, combined with the preferred sensor design,allows for such rapid measurements of heat capacity curves anddeterminations of phase transition points. A measurement of the heatcapacity at a given temperature requires measuring the modulationamplitude at that temperature. An accurate measurement of the modulationamplitude typically requires averaging or Fourier transforming over atleast several cycles. Five cycles, for example, is a reasonable minimumnumber. Thus, at a given temperature, an accurate determination of theheat capacity can be made in approximately 0.1 seconds for a typicaltemperature modulation frequency of 2ω=50 Hz. If it is desired to obtainone measurement per degree as the temperature is varied, for example,then the average temperature may be varied at a rate of approximately10° C. per second, or 600° C. per minute, compared to typical sweeprates of 10° C. per minute for conventional DSC instruments.

[0115] In practice, the temperature is not stabilized at a set ofdiscrete values while measurements are made at these values; rather, thetemperature increases continuously, and the modulation data can beconsidered a “running average” of the modulation amplitude over a finitetemperature range. This temperature range is typically several degrees,and is determined by the temperature sweep rate and the averaging timefor the modulation amplitude measurement.

[0116] Referring to FIG. 12C, using the 2ω method in the sensor arraystructure according to the present invention does not require anymodification of the sensor structure itself because of the modularsensor array structure, standardized interconnection method, andflexible electronic platform. As explained above, each sensor 12 in thesensor array 10 is connected to a multiplexer 126 or other signalrouting means, and both the multiplexer 126 and the electronic testcircuitry 127 for driving the sensors are controlled by a computer 128.The electronic test circuitry 127 and the computer 128 together can beconsidered a flexible electronic platform. To characterize materials onthe sensors 12 one at a time, the computer 128 controls the multiplexers126 so that it connects a given sensor 12 the electronic test circuitry.The electrical signals for a complete scan (as selected by the user) aresent to and read from the heater 104 and thermometer 102 on the selectedsensor 12, and then the multiplexer 126 switches the connection to linkthe electronics platform with the next sensor in the sequence. Thus, theinvention allows for ultimate flexibility in sensor array testing.

[0117] Sample results from a test conducted according to the 2ω methodare shown in FIGS. 12E through 12H for illustrative purposes only. Morespecific details on the preferred manner in which the tests areconducted are as follows: The heater ramp voltage is obtained from anauxiliary analog output of an Stanford Research Systems SRS 830 lockinamplifier. This voltage is set via instructions to the lockin amplifierfrom the computer, transmitted over a GPIB interface. For the specificheater 104 design in the preferred sensor embodiment, a voltage rampfrom 0 to 1.5 volts is sufficient to raise the temperature of the sensorto approximately 150° C. (in vacuum). Higher maximum voltages result inhigher maximum temperatures. The ramp voltage is incremented by a smallamount (approximately ten times per second) and the size of theincrement can be specified by the user before beginning a scanningoperation. The size of the increment is typically in the range of fromabout 0.005 to 0.01 volts, so the total scan time is approximately 15 to30 second. When the maximum voltage is reached, the ramp voltage iseither ramped back down to zero at the same rate while taking data; orthe ramp voltage is set to zero and the scan is completed.

[0118] The heater modulation voltage is generated by the same lockinamplifier's sine wave oscillator output. Fundamental frequencies of10-40 Hz were generally used with the 2ω method, and a typicalmodulation amplitude is several tenths of a volt. The ramp andmodulation signals are added by a summing amplifier from OpAmp Labs,which also buffers the signals and supplies adequate current to drivethe heater, which has a 2-wire impedence of approximately 100Ω.

[0119] The DC current for the thermometer 102 was generated byconnecting a 9V battery in series with a 10 kΩ resistor and thethermometer 102, producing a current of approximately 1 mA. The use of abattery-powered current source insures that the thermometer 102 circuitis isolated from ground and from the circuitry connected to the heater104. The thermometer 102 resistance is measured in a 4-wireconfiguration, and the 4-wire resistance at room temperature istypically 50Ω. Thus, the initial thermometer voltage is approximately 50mV.

[0120] The thermometer voltage is then analyzed to extract the averagevalue, which gives information on the temperature, and the modulationamplitude, which gives information on the temperature oscillationamplitude and the heat capacity. To measure the average thermometervoltage, the thermometer voltage is connected to the differential inputsof an SRS 560 low-noise voltage preamplifier with variable gain and aprogrammable filter. The preamplifier is typically used with a voltagegain of approximately 10, and a low pass filter set at 1-3 Hz to removethe modulation. The preamplifier output is connected to an auxiliaryanalog-to-digital converter input of the lockin amplifier, and thevoltage is read via instructions from the computer.

[0121] To measure the modulation voltage, the thermometer voltage issent to the differential inputs of the SRS 830 lockin amplifier, whichis set for signal detection at the second harmonic frequency of the sinwave being output from the oscillator. The second harmonic frequency isthus typically in the range 2f=20-80 Hz. The lockin input bandpassfilter is set at 24 dB/octave, and a 0.3 second output time constant istypically used. Although phase-sensitive detection can easily be done,only the total magnitude of the modulation signal was recorded forsimplicity. This is permissible if the frequency is properly chosen sothat t1>>π/ω>>t2, where t1 and t2 are the external and internal thermalrelaxation times discussed above.

[0122] The correct measuring frequency is chosen by measuring themodulation voltage V_(th)(2ω) as a function of the drive frequency, andlooking for a broad peak in a plot of ω*V_(th)(2ω) versus f=ω/2π, as iswell known to those skilled in the art of AC calorimetry. An example isshown in FIG. 121, using the preferred sensors discussed above. At lowfrequencies ω<<π/t1, the temperature modulation amplitude ΔT andV_(th)(2ω) are independent of frequency, since a balance always prevailsbetween the modulated heat input and the losses to the environment. Inthis region, ω*V_(th)(2ω) increases linearly with Ω. In the optimalfrequency range for conducting calorimetry measurements, ΔT isproportional to 1/ω, as explained previously, so ω*V_(th)(2ω) isapproximately constant. At high frequencies ω>>π/t2, the thermometertemperature is out of equilibrium with the heater temperature, sincethere is insufficient time for heat to diffuse across the width of thethermometer during a single cycle. The temperature distribution over thethermometer takes the form of a damped travelling wave, with awavelength shorter than the size of the thermometer, and the averagetemperature and voltage modulation decrease as the frequency isincreased above π/t2. Thus a plot of ω*V_(th)(2ω) has the form of apeak, with the broad maximum indicating the optimum frequency range forperforming calorimetry measurements. Because of the breadth of the peakin this plot, it is not necessary to perform a frequency analysis foreach sample. Once it has been done for a given type of sample (e.g. aclass of materials with roughly similar film thickness and thermalconductivity), the same frequency can be used for all subsequentmeasurements on samples of that general type. In the example, 15 Hz ispreferred, but anywhere in the range of from about 5 to about 30 Hz maybe used.

[0123] Once the modulation frequency and amplitude have been set and allof the signals are properly routed, numerous measurements can be rapidlymade using a simple procedure. In the preferred embodiment of theprocedure, a group of sensors is first specified for measurement viamanual input to the computer by the user. Once this has been done, theuser instructs the computer to begin an automated measuring procedure.All operations described below are performed automatically by thecomputer, following parameters set by the user before the automatedprocedure is begun, such as setting the ramp rate, etc.

[0124] The computer closes selected switches in the multiplexer so thatthe first sensor in the specified set is connected to the electronicsinstruments (current source, lockin, oscillator, etc.). It is desirableto wait several seconds for the electronics to settle after closing theconnections between a sensor and the electronic platform, to eliminatetransient responses.

[0125] The heater ramp voltage is initially zero. Before beginning toincrease the ramp voltage, the computer records the average thermometervoltage, which is defined as V(T₀). T₀ is the temperature of the sensorat the beginning of the scan. This will be somewhat higher than roomtemperature due to the power dissipated in the heater by the modulationvoltage. In a preferred procedure, the modulation voltage is also set tozero before V(T₀) is recorded. It can then be assumed that To is equalto the room temperature, provided that the heat dissipated in thethermometer 102 by the DC current causes only minimal self-heating.Various other procedures may be performed to determine more preciselythe sensor temperature at the beginning of the scan.

[0126] The modulation voltage is then turned on again, and the followingprocedure is iterated or looped approximately ten times per second: (1)measure the average thermometer voltage <V_(th)> and the modulationvoltage V_(th)(2ω); (2) calculate the temperature T using the formulaT=[<V_(th)>/V_(th)(T₀)−1]/α+T₀, where the coefficient α ischaracteristic of the metal which the thermometer is made out of and canbe determined separately by a variety of well known means. For Pt,typically α=0.0025−0.003; (3) calculate a quantity proportional to theheat capacity, referred to as the “heat capacity signal” C_(p), usingthe formula C_(p)=[<V_(th)>*V_(th)(2ω)]⁻¹, as discussed above; (4) storethe values of the time, the heater drive voltage V_(H), and the measuredand derived quantities <V_(th)>, V_(th)(2ω), T, and C_(p) in computermemory; (5) increment V_(H)to a new value; and (6) repeat steps (1)through (5). When the scan is finished, V_(H)is set to zero and the datastored in memory are transferred to a file on a storage device. The nextsensor is then selected by the computer and multiplexer, and the entirescan procedure is repeated.

[0127] An alternative AC calorimetry method that can be used in theinvention is the “3ω” method. FIGS. 13A and 13B are representativediagrams explaining the preferred materials characterization apparatusconfiguration using the 3ω method, while FIGS. 13C through 13F showexamples of input and output signals according to this method. In thismethod, the heater receives only a ramped DC voltage V_(H,0)(t), insteadof a ramped voltage with a modulated AC voltage superimposed thereon.Also in the “3ω” method, an AC current in the form of a pure sine waveat frequency ω is sent through the thermometer instead of a DC current.The AC current through the thermometer preferably has a constantamplitude. Further, rather than measuring the 2ω modulation amplitudeand the average value of the thermometer voltage to determine the samplematerial's heat capacity and temperature, respectively, the 3ω methodmeasures the third harmonic in the thermometer voltage to determine thesample's heat capacity, as shown in FIG. 13E, and measures the firstharmonic voltage to determine the temperature, as shown in FIG. 13D andas explained below.

[0128] If the AC current amplitude is sufficiently small, or thesample's heat capacity is sufficiently large, then the temperature ofthe sample does not vary in response to the AC current. The thermometerresistance is constant, and the thermometer voltage is a pure sine wave,since V_(TH)=IR_(TH) and I is a pure sine wave. In this case there areno higher harmonic signals. If the AC current is sufficiently large,however, the input power modulation at frequency 2ω will cause atemperature modulation, and therefore a resistance modulation, atfrequency 2ω, i.e., R_(TH)(t)=<R_(TH>+ΔRe) ^(2iωt), where ΔR isproportional to ΔT. Since V_(TH)=IR_(TH) and I is a pure sine wave,V_(TH)=I₀e^(iωt)R_(TH)(t)=I₀<R_(TH)>e^(iωt)+I₀ΔRe^(3iωt). The firstharmonic voltage is thus proportional to the thermometer resistance, andtherefore to the temperature, while the third harmonic voltage isproportional to the temperature modulation, and therefore givesinformation about the heat capacity, as in the 2ω method.

[0129] Typically, the ω component of V_(TH) is between 100 and 1000times larger than the 3ω component, depending on the sample's particularthermal characteristics, the AC signal amplitude, and the geometry ofthe heater/thermometer 100. To analyze the voltage output from theheater/thermometer 100, a component in the electronic platform thatreceives the voltage output can lock in at frequency ω to detect thebasic sine waveform and at frequency 3ω to detect the third harmonic. Asexplained in FIGS. 13A and 13B, two separate lockin amplifiers 130, 132or a single lockin amplifier that can switch between the two frequenciescan be used. The advantage of using two separate lockin amplifierstuned, respectively, to the ω and 3ω frequencies 130, 132 is that boththe temperature and the heat capacity measurements can be conductedsimultaneously in real time, greatly increasing measurement speed andeliminating the waiting period needed for a single lockin amplifier tosettle after switching its frequency. A representative block diagramillustrating the components of a preferred sensor array and electronicplatform for the 3ω method is shown in FIG. 13B.

[0130] The 3ω method in its preferred embodiment requires additionalsignal processing equipment or methods in order to extract separatelythe modulation amplitudes at two separate frequencies. However, the 3ωmethod has a number of advantages over the 2ω method as well. In the 2ωmethod, the power modulation is produced by the heater 104, while thetemperature modulation is sensed at the thermometer 102. The timeconstant t2 is the thus time required for heat to diffuse laterallyacross the membrane 94 from the heater 104 to the thermometer 102. Whilethis time can be made fairly small, this still limits the frequencyrange to typically 5-50 Hz, and therefore places some limits on themeasurement speed.

[0131] In the 3ω method, the temperature modulation is both produced andmeasured by the thermometer; in this case, t2 is the time required forheat to diffuse vertically across the thickness of the membrane 94 andinto the thin film sample 90 rather than horizontally from the heater104 to the thermometer 102. Because the sample 90 and the sensor 12taken together are typically only a few microns thick, this time is muchshorter than the t2 associated with the 2ω method. This in turn permitsthe use of measuring frequencies in the kHz range, with a correspondingincrease in the possible temperature ramp rate and measurement speed. Inaddition, because the modulated power does not have to diffuse anydistance laterally across the membrane, there are no radiative losses asthe power travels from the modulation source to the modulation sensorsince they are one and the same.

[0132] Sample test results obtained using the 3ω method are shown inFIGS. 13C through 13F. The samples are a film of low Tg solder glass,form Ferro, as detailed above. The configuration of the electronicsplatform for the 3ω method is somewhat different than for the 2ω method,but once the configuration is completed, the measurement procedure isessentially the same as described above with respect to the 2ω method.The heater ramp voltage is generated in the same way as in the 2ωmethod, but instead of being summed with a modulation signal, it issimply buffered and sent to the heater 104. The modulation signalcontains a sinusoidal AC current and is sent to the thermometer 102instead of the DC current used in the 2ω method. The AC current can beproduced in many ways. For the example discussed here, the sinusoidalvoltage output from a lockin amplifier's oscillator output is used asthe input to a voltage-controlled current source, which is a simpleop-amp circuit. The amplitude of the modulation current is typicallyseveral tens of mA in order to get an adequate third harmonic signal dueto temperature modulation.

[0133] The thermometer 102 is connected in parallel to the differentialinputs of two separate lockin amplifiers 130, 132. A ω lockin amplifier130 is set to detect signals at the same frequency as the oscillatordriving the AC current source, while a 3ω lockin amplifier 132 is set todetect signals at the third harmonic of this frequency. The oscillatoroutput from the lockin 130 used to drive the AC current source is alsoconnected to the reference input of the second lockin 132, insuring thatboth lockins 130, 132 are synchronized and tuned to the correctreference frequency and phase. As noted above, the signal at 3ω istypically 100-1000 times smaller than the signal at ω (e.g., 10 μV vs.10 mV), so a much higher gain setting is used for the lockin which ismonitoring the third harmonic. Because the 3ω lockin 132 must reject themuch larger first harmonic signal, it must usually be used in “highdynamic reserve” mode, instead of “low noise” mode as is possible in the2ω method, in order to avoid overloading the inputs.

[0134] Once the measurement has been configured, the same measurementprocedure used in the 2ω method can be used in the 3ω method. In thiscase, V_(th)(ω) corresponds to the resistance of the thermometer 102 andthe average temperature; and V_(th)(3ω) corresponds to the temperaturemodulation and heat capacity, as explained earlier. The temperature iscalculated from V_(th)(ω) in the same way as described above for the 2ωmethod using <V_(th)>. However, the heat capacity is approximated asCP=V_(th)(ω)/V_(th)(3ω), which differs in form from the formulaCp=[<V_(th)>*V_(th)(2ω)] used with the 2ω method.

[0135] The reason is again related to the formula C_(P)=ΔP/2ωΔT,discussed earlier. In the 3ω method as described here, the modulation isdriven by a sinusoidal current of fixed magnitude I_(th), and the powermodulation in the thermometer ΔP is given by I_(th) ²R_(th), which isproportional to R_(th) and therefore to V_(th)(ω). In the embodiment ofthe 2ω method described earlier, the modulation power to the heater wasdue to a modulation voltage of fixed amplitude v_(H). The modulationpower is then v_(H) ²/R_(H), and is inversely proportional to the heaterresistance.

[0136] It should be noted that both AC and DC power can be coupled intothe heater 104 and/or thermometer 102 in either the 2ω or 3ω methods,and the temperature and temperature modulation may be determined bymonitoring either the voltages on the sensors caused by known currents,or the currents caused to flow by known voltages. It should also benoted that other implementations of AC calorimetry in a sensor arrayformat are possible, without departing from the spirit of the invention.For example, two separate heaters and one thermometer can be used,wherein one heater provides a DC power input and the other heaterprovides an AC power input. Also, a single resistive element can be usedas both a heater and thermometer if the 3ω method is used and theresistive element is properly designed so that the temperature issubstantially uniform over the area of the thermometer. An example ofsuch a sensor design is shown in FIG. 13G. Although the sensor consistsof a single wire, with uniform current passing along its entire length,the voltage is only measured from a portion of the wire, which is in thecenter of the area being heated. A combined DC and AC current is used,and the voltage may have frequency components at all harmonics up to thethird. As in the previous description above, the temperature and heatcapacity may be obtained from the first and third harmonics,respectively. This sensor design has the advantage that both AC and DCpower are created uniformly across the entire sensor.

[0137] Further, the temperature of the sensor can be varied via anexternal heating method, such as contact with a heated block orillumination by infrared radiation, while the temperature andtemperature modulation are measured electronically by the temperaturesensor 102.

[0138] Although the preferred substrate for thermal analysis is a filmhaving a thickness comparable to the thickness of the sample, the use ofmodulation techniques, such as the 3ω method, also permits thermalanalysis of films on substrates that are much thicker than the sample.In such a case, the modulation frequency must be sufficiently high sothat the distance over which heat diffuses into the substrate during onemodulation cycle is comparable to or less than the sample thickness.This distance defines the effective sampling depth of the modulatedcalorimetry measurement, and so the heat capacity contributions from thesample and substrate will be comparable, even though the total heatcapacity of the substrate is much larger. The 3ω method is particularlyuseful in this case because it can access much higher measuringfrequencies than the 2ω method.

Experimental Example: Thermal Stability Analysis

[0139] The thermal analysis array structure explained above can also beused to measure the thermal stability of a material. Thermal stabilitymeasurements indicate how hot a material can get before it decomposes orvaporizes and how quickly decomposition takes place as its temperatureincreases. Thermal stability is particularly important when determiningwhether a particular material can withstand high temperatures withoutbreaking down or otherwise exhibiting volatile properties.

[0140] Thermal stability can be measured in several ways. FIG. 14 showssample results of a glass transition and thermal decomposition of apolystyrene film using the sensor array of the present invention. Thepolystyrene was obtained from Aldrich and used as obtained, which wascatalog no. 43,010-2 having a listed melt index of 8.5 and a molecularweight of 230,000. The polymer sample was dissolved in toluene to createa 3% solution, which was manually pipetted onto the sensor. The samplewas allowed to air dry on the sensor and then placed in a chamber thatwas evacuated for a measurement using the 3ω method, described above.Thermal stability measurements can be conducted via any of the signalmodulation methods described above. The measurement is conducted in anidentical manner as the heat capacity measurement, but the temperatureis increased until the material decomposes or otherwise gives up mass.When this occurs, the heat capacity drops sharply and the modulationamplitude increases sharply. This occurs because the same amount ofmodulated power is going into the heater/thermometer 100 and themembrane 94, but the sample has partially or largely disappeared, so themodulation becomes larger. Further, because the change in the materialis not reversible, the modulation will remain large even if thetemperature is lowered because of the material's irreversibly changedstate.

[0141] Heat capacity and thermal stability measurements conducted inthis manner are most appropriate for materials that do not liquefyexcessively when exposed to heat, such as high molecular weightpolymers, because materials having extremely low molecular weights maynot stay on the heater/thermometer 100 when heated and may tend to runto the edges of the sensor 12, leaving the heater/thermometer 100exposed. As a result, the exposed heater/thermometer 100 will give afalse indication of decomposition (e.g. a large increase in modulation),because much of the material is no longer on the sensor 12, when inreality the material has simply liquefied and flowed off theheater/thermometer 100. Thus, the thermal capacity measurementsdescribed above are more suitable for materials that tend to hold theirshape rather than low viscosity liquids. Thermal stability can also bemeasured with the present invention by heating the sample material onthe heater/thermometer 100 in a chamber until it burns and decomposes,then measuring the amount of gaseous fragments in the air as well as thefragments' mass and the air pressure within the chamber.

Dynamic Thermal Analysis

[0142] Dynamic thermal analysis may be a less quantitative technique foridentifying phase transitions. A sample is typically placed in a cell incontact with a heater block. One thermometer monitors the temperature ofthe sample, while another thermometer measures the temperature in areference cell or reference location. The difference in the temperaturesof the two thermometers is measured as the temperature of the heaterblock is steadily raised. The sample temperature tends to lag behind thereference cell temperature, in proportion to the heat capacity of thesample. Thus, phase transitions, such as glass transitions or meltingpoints, show up as kinks or bumps in the temperature vs. time curve.

[0143] A preferred structure for conducting dynamic thermal analysis ina sensor array according to the invention is shown in FIG. 15A. Thestructure has a heater block 150 constructed from a block of materialhaving good thermal conductivity, such as copper or another metal. Thehigh thermal conductivity of the block material causes the heater block120 to have and maintain a uniform temperature throughout even while theheater power and temperature are varied.

[0144] The preferred structure also includes a glass plate 152 that isplaced on top of the metal block. Glass is the preferred material forthicker substrates because of its relatively low cost, rigidity, and lowthermal conductivity. A plurality of temperature sensors 154, areprinted on the top surface of the glass plate 152 in any desired arrayconfiguration using any known method, such as lithography. Because glasshas very poor thermal conductivity, there will be a relatively largedifference between the top and the bottom surfaces of the glass plate152.

[0145] For clarity, the following discussion will describecharacterization of a single material, but a library of materials can besimultaneously and/or selectively characterized on the sensor array. Themain principle behind dynamic thermal analysis is that the temperaturedrop across the thickness of the glass plate or between twopredetermined points on a surface of the glass plate is proportional tothe heat flow through the glass into the sample. Although this ignoresthe heat flow that is absorbed by the glass to raise its temperature,this heat absorption is the same at all locations on the sensor arrayand can effectively be disregarded. For materials characterization, asillustrated in FIG. 15A, a sample material is placed on a sensor and thetemperature of the heater block TO is increased to supply heat throughthe glass plate 152 to a sample 156. In this particular example, thetemperature increase in the heater block 150 will create a temperaturedifference ΔTij across the thickness of the glass 152, and the heat willeventually conduct through the glass to heat the sample up to atemperature Tij. Thus, the temperature difference across the glass plateΔTij=Tij−T0. Alternatively, the temperature difference can be measuredbetween a point on the glass plate 152 containing the sample and areference point on the glass plate 152, which reference point maycontain a sample that is known to not have any phase transitions in thetemperature range of interest.

[0146] The sensors 154 on the glass plate 152 measure the temperatureand temperature increase rate of each sample. If the temperature of thesample 156 is rising at 1 degree per second, for example, there must bea certain amount of heat flowing through the glass 152 to the sample156. When the temperature of the heater block 150 is ramped upward, acertain amount of heat flow J is required to increase the sample'stemperature at the same rate. If there is not enough heat supplied tothe sample 156 to raise its temperature, the temperature of the sample156 increases mores slowly than the heater block 150, increasing thetemperature difference ΔTij between the top and bottom surfaces of theglass 152. As the temperature difference increases, more heat flowsthrough the glass 152. For each material, there is a specific value ofΔTij at which the heat flow through the glass 152 is the correct, steadystate amount for raising the temperature of the material sample. Becauseeach material has different thermal characteristics, the heatertemperatures at which this steady state condition occurs, and thus theΔTij value, will be different for different materials.

[0147] The temperature difference ΔTij corresponds qualitatively to theheat capacity of the sample 156 because some materials require a greaterheat input to raise its temperature a certain amount and thereforecauses a higher value for ΔTij. As a result, a large ΔTij correspondsqualitatively with a higher heat capacity material, while a lower ΔTijcorresponds to a lower heat capacity material. More importantly, thelarge changes in the heat capacity, which occur at phase transitions,will show up as kinks or bumps in the temperature vs. time data for agiven sensor. For example, the temperature difference ΔTij between thetop and bottom surfaces of the glass plate 152 increases sharply at amelting point because large increases in the heat input result in littleor no change in the sample material's temperature; the temperatureincrease in the sample material lags behind the temperature increase inthe heater block 150 by a much larger amount than at a point away fromthe melting point of the sample 156. After melting is complete, ΔTij mayreturn to a lower value.

[0148] The structure for dynamic thermal analysis is particularlysuitable for testing materials that cannot dissolve easily in a liquidand form a thin film on the sensor when the liquid evaporates, such ashighly crystalline polyethylene samples. For dynamic thermal analysis,as explained above, the sample material can be simply dabbed onto eachsensor without having to form a thin film, e.g., from a slurry, gel, orpowder. Further, the thermal characteristics of the glass plate 152 inthe present embodiment do not adversely affect the thermalcharacterization procedure if the dimensions of the material sample andthe glass plate's thickness are on the same order of magnitude.

[0149]FIG. 15B is a representative block diagram of a materialscharacterization apparatus that is designed for dynamic thermalanalysis. As explained above, an insulating substrate, such as a glassplate, has a plurality of thermometers 154 disposed on its surface andsits on top of a metal block heater. The temperature of the metal blockheater is increased, and the electronic platform monitors thetemperature of the block with one or more thermometers that are incontact with the block.

[0150] Because the entire sensor array structure is heatedsimultaneously in this particular example, all of the samples on thesensor array must be measured simultaneously or via rapid repeatedscanning. Therefore, the preferred electronic link between the sensorarray and the electronic platform will include multiple channels formonitoring the sensor operation, preferably one channel per sensor, formaximum speed. Alternatively, the electronic platform rapidly scansthrough all of the sensors via the multiplexer to measure each sample'stemperature (by measuring the resistance in each thermometer). Thetemperature difference ΔTij can be calculated by the computer orprocessor, if desired, to generate the thermal characterization dataand/or plot. The reference temperature can be the temperature of theheater block 120, or the temperature of a sensor that does not carry asample or that carries a sample having no phase transitions over thetemperature range being studied.

Experimental Example: Dielectric Spectroscopy

[0151] The sensor array of the present invention is not limited only toconducting thermal analysis. As illustrated in FIGS. 16A through 16D,for example, the invention can also characterize electrical properties,including but not limited to the complex dielectric constants ofmaterials.

[0152] The basic principles behind dielectric spectroscopy are nowbriefly discussed. To measure the dielectric constant of a material, thematerial is typically placed in between two metal plates that have anelectric field in between them going from a positive charge to anegative charge. If the molecules of the material in between the twometal plates are more asymmetric, they usually have a greater tendencyto polarize in response to the electric field, and the molecules willrotate so that they align with the electric field. The molecularrealignment of the material creates its own electric field responsive tothe electric field imposed on the material and tends to cancel out atleast part of the imposed electric field. Materials having strongerdipole characteristics (and therefore a greater dielectric constant)will create a stronger responsive electric field and will thereforecancel out a greater portion of the imposed electric field.

[0153] The overall electric field reduction can be measured bymonitoring the charge Q required to create a voltage V between the twometal plates. When the material to be tested is placed between the metalplates, an additional charge (ε−1)Q may flow onto the plates to maintainthe voltage V, wherein ε is the dielectric constant of the material. Ascan be seen from the equation, a material with a larger dielectricconstant will require more charge to achieve a given voltage drop acrossthe metal plates. In short, the plates and the material together form acapacitor, and changes in the capacitance reflect changes in thedielectric constant.

[0154] The dielectric constant provides information about the physicalcharacteristics of the material being tested at the microscopic level.Some molecules whose positive and negative charges are at the center ofeach atom in the molecule will exhibit dielectric properties when placedin the electric field because the electric field will slightly displacethe nuclei of the atoms in the molecules, creating a positive charge atone end of molecule and a negative charge at the other end. Materialsthat exhibit greater dielectric properties, however, often havemolecules that are asymmetrically charged to begin with. When thematerial is placed in the electric field, the molecules simply rotateand align themselves with the electric field.

[0155] Monitoring the dielectric properties of materials over time is aneffective way to detect, for example, curing or cross-linking of glues,thermosets, epoxies, and similar adhesive materials. FIG. 16Billustrates an example where the dielectric properties of a 5-minuteepoxy are monitored over time using the sensors described below. In atypical epoxy curing reaction, the molecules in the liquid resininitially move and rotate relatively freely, allowing them to orient inresponse to an imposed electric field. As the molecules begin tocross-link (e.g., thereby hardening the epoxy or glue), they are lessable to align themselves in response to the electric field, decreasingthe dielectric constant of the material and thereby decreasing thesensor's capacitance. After the epoxy is completely cured, the moleculesare not able to realign themselves, dropping the dielectric constant ofthe material, and therefore the capacitance of the sensor down evenfurther. Thus, monitoring changes in the dielectric constant of amaterial over time can provide valuable information about the speed andnature of chemical reactions, such as the epoxy curing reactiondescribed above.

[0156]FIGS. 16A, 16C, 16D and 16E explain the preferred sensor geometryand apparatus structure that can be used for dielectric spectroscopy inthe sensor array of the present invention. For simplicity, the structureand operation of only one sensor is discussed, but, like the otherexperimental examples, the preferred method and apparatus for conductingtesting involves using a plurality of sensors disposed on a sensor arrayand coupled with an electronic platform, as represented in FIG. 16E. Themost common technique for measuring the dielectric constant of amaterial is, as noted above, forming a capacitor with the material to betested in between two plates. Forming a sandwich-type capacitor andobtaining measurements from such a capacitor, however, is often acumbersome operation, especially when used with liquid samples.Furthermore, the geometry of the capacitor needs to be well defined; theuser should know the exact thickness of the capacitor layers, positionthe plates, and maintain these dimensions throughout the testing.

[0157] As shown in FIGS. 16C and 16D, a preferred sensor structure fordielectric measurement according to the present invention is a planarcapacitor having interdigitated electrodes 160. The interdigitatedelectrodes look somewhat like two interlocking combs where the “teeth”162 do not touch each other. Note that because the thermalcharacteristics of the substrate 164 are not a concern when measuringelectrical properties, the substrate supporting the electrodes 160 canhave any thickness (i.e., it does not have to be a thin membrane).However, it is desirable that the substrate 164 should have lowdielectric losses under the desired measurement conditions and notexhibit any phase transitions or other unusual behavior. Thus, theelectrodes 160 themselves can be printed on glass sheet, quartz,sapphire, or any other desired inert substrate material.

[0158] The advantage of the interdigitated electrodes is that thematerial sample's dimensions do not affect the output of the sensorbecause the capacitors formed by the wires of the interdigitatedelectrodes 160 are so small; as long as the thickness of the materialplaced on the electrode 160 is a few times larger than the spacingbetween the electrode wires, the thickness of the material sample is nolonger important because the electric field is virtually zero at adistance that is several times the spacing between the wires 162. Forexample, if the spacing between wires 162 in the electrodes is 5microns, the electric field is reduced roughly by a factor of 10 forevery 10 μm of distance away from the surface. The wire spacing ispreferably kept as small as possible so that the capacitance can be keptlarge enough to measure easily. More particularly, the capacitanceobtained from a given sensor will be in the range of L²/D (in picofaradspF), where L is the length of one side of a square sensor and D is thespacing between the wires, both in units of centimeters. For thisexample experiment the electrode 160 dimensions for use in the sensorarray of the present invention was a 2 millimeter square sensor with a 5micron wire spacing, which will give capacitance readings of around10-15 pF. However, the electrodes 130 can any have dimensions to obtaina capacitance range meeting the user's own specifications.

[0159] For example, the sensor array used in the experiment shown inFIG. 16B was fabricated from 1000 Å Cr metal on a 5″ square glasssubstrate using a standard photomask blank as a starting substrate. Thestarting substrate is preferably purchased pre-coated with the metal anda photoresist. The photoresist was patterned by contact printing from amaster photomask, and the exposed Cr metal was etched away chemically.The resulting interdigitated electrodes 130 cover a 2 mm square andcontain lines and spaces of 5 μm.

[0160]FIG. 16E is a simplified block diagram representing a materialscharacterization system having a sensor array that is designed formeasuring dielectric properties. Like the other embodiments describedabove, the sensor array is controlled by an electronic platform via amultiplexer that directs electronic signals to and from selectedsensors. The electronic platform can measure the complex impedenceacross each capacitor to determine the capacitance, resistance andcomplex dielectric constant of the materials on each sensor. Forexample, the capacitance of a sensor can be measured in less than 0.1seconds using a conventional capacitance/resistance meter or impedanceanalyzer. The multiplexer can scan the electrodes 160 in any order andany combination rapidly, as explained in previous examples.Alternatively, a separate drive circuit can be provided for each sensorso that the sensors can be measured simultaneously. The capacitance andlosses due to the various interconnect circuitry, including wires,signal routing means, etc., can be measured with the sensor array 10removed from the apparatus. Subsequent measurements with a sensor array10 in place can then be corrected to separate the impedance of theelectrodes 160 from the impedance due to the interconnects.

[0161] Because dielectric spectroscopy does not necessarily involvemeasuring the thermal properties of the material, monitoring thematerial temperature is not necessary if measuring the dielectricconstant alone. However, the interdigitated electrode structure can becombined with, for example, a resistance thermometer 166. This combinedstructure can monitor changes in the dielectric constant during achemical reaction while simultaneously monitoring thermal events such asexotherms. The combined electrode/thermometer structure preferably hasthe thermometer placed in the center of the electrode to provide themost accurate temperature reading. By conducting the dielectric andthermal measurements simultaneously, more information is made availablefrom a single experiment. In addition, the glass transition of the curedresin can be measured by operating the system in Dynamic ThermalAnalysis modes as described above.

[0162] More specifically, the scans shown in FIG. 16B were obtained bycoupling selected sensors to an SRS 560 LCR meter(inductance/capacitance resistance meter) operated at a 1 kHz frequency.Although one preferred operation mode includes repeated measuring thecapacitances of multiple sensors in the array 10 during a singleexperiment, the data shown in FIG. 16B was acquired one sample at atime. The LCR meter was coupled to a selected sensor's contact pads 14via two wires attached to micropositioners. Once the sensor wascontacted, a fluid sample was applied directly to the sensor, andmeasurements were recorded manually once per minute, for a total time often minutes. The capacitance of the leads, before connecting them to asensor, was approximately 1 pF. The capacitance of a bare sensor withthe leads was approximately 15-20 pF. The capacitance of a sensor withone of the epoxy components placed on top of it was typically 30-40 pFimmediately following application of the sample.

[0163] The epoxy used in the specific example shown in FIG. 16B isDevcon 5-minute epoxy. In the experiments conducted on the individualepoxy components, denoted A and B in the figure, fresh samples of thecomponents were removed from the storage tube immediately before beingapplied to the sensor. When a mixture of A and B was tested, the twocomponents were removed from their tubes and mixed for approximately 30seconds before being applied to the selected sensor. A large reductionof the sensor capacitance can be seen for the sample of the mixed epoxy,corresponding to setting, while the capacitance for the individualcomponents A and B change by much smaller amounts.

[0164] Determining the dielectric properties of materials in and ofthemselves can also be important. For example, integrated circuits oftenincludes dielectric layers separating multiple wires from each other tominimize or eliminate cross-talk, and it has been found that lowerdielectric constant materials, which do not polarize easily, allowsignals to propagate more quickly. Thus, conducting dielectricspectroscopy according to the claimed invention allows rapid screeningof many materials to find materials that have the optimum dielectricproperties.

Experimental Example: Surface Launched Acoustic Wave Sensors

[0165]FIGS. 17A and 17B show an example of a surface launched acousticwave sensor 170 for measuring material properties such as viscosity,density, elasticity, and capacitance. An electrode in the surfacelaunched acoustic wave sensor may also have interdigitated fingers 172,in this case for launching and measuring transmission of acousticenergy. Further, like the examples shown in FIGS. 16C and 16D anddescribed above, the interdigitated structure of the sensor electrodesin FIGS. 17A and 17B can measure the dielectric constant and theconductivity of the material, if desired.

[0166] In this example, surface launched acoustic wave sensors can befabricated on thin silicon-nitride or etched silicon membranes 174similar to those described above. A piezoelectric material 176, such aszinc oxide, is then deposited as a thin layer on top of the membrane toproduce an acoustic wave sensing device. The physical dimensions of theelectrode, such as its thickness, size, and configuration, can beadjusted so that the electrode operates in, for example, a surfaceacoustic wave (SAW) resonance mode, a thickness shear mode (TSM), aflexural plate wave (FPW) resonance mode, or other resonance mode. Whenthe electrode acts as a resonator, its resonating response is affectedby, for example, the sample's viscosity and density. U.S. applicationSer. No. 09/133,171 to Matsiev et al, filed Aug. 12, 1998, describesmechanical resonators in more detail and is incorporated by referenceherein.

[0167] Thus, because the surface launched acoustic wave device shown inFIGS. 17A and 17B can serve as both a mechanical resonator and as asensor for characterizing other material properties, such as thedielectric constant, multiple devices can be arranged in a sensor arrayformat to screen material properties. The versatility of the surfacelaunched acoustic wave device and other mechanical resonators make it agood choice for observing multiple materials as they undergo a chemicalor physical process involving changes in viscosity, density,conductivity, molecular weight, or chemical concentration.

[0168] Further, the mechanical resonator can be used to measure weightor force because of its responsiveness to mechanical loading. Whensurface launched acoustic wave sensors are arranged in an array format,the sensor array can be used to weigh simultaneously multiple samples ofpowders or liquids, each sensor acting as a separate scale. As notedabove, these mass measurements can be conducted simultaneously withviscosity, conductivity, and dielectric constant measurements due to theinterdigitated structure of the sensor electrode. Electroplating orsolution deposition could also be measured using the arrayed mechanicalresonators by correlating the resonator response to mass loading.

[0169] The mechanical resonator structure shown in FIGS. 17A and 17B canalso be used for magnetic material characterization. More particularly,the mass loading effect of the resonator can be used to measure a samplematerial's response to an external magnetic field. In this application,the resonators in the array are coated with the test material ormaterials, and the sensor array is placed in a magnetic field. Anymagnetic response of the material to the magnetic field would appear asa change in the mass loading experienced by the resonator. This massloading change damps the resonance signal from the resonator, and theamount of damping can be correlated with the material's response to theapplied field. Alternatively, a mechanical actuator can be used in thesame manner as the mechanical resonator, and characterization would beconducted by measuring the amount of displacement in the actuator.

Experimental Example: Electrical Transport Properties

[0170] Yet another set of material properties that can be measured usingthe materials characterization system of the present invention are theelectrical transport properties of materials: electrical resistance,Hall effect resistance, magnetoresistance, and current-voltage curvesshowing non-linear features such as breakdown voltages and criticalcurrents. As explained above, the “plug-and-play” format of theinvention may only requires the user to change the sensor array, not theentire machine or any hardware, to measure a different materialproperty, depending on the embodiment being practiced. The changes mayoccur at the sensor level; the electronic platform and multiplexer canremain generally the same regardless of the specific materialcharacteristic to be tested. Minor variations in the electronichardware, such as amplifiers, voltmeters, capacitance meters and thelike may be needed to conduct the measurements, but these modificationscan be external to the sensor array and can be conducted in the flexibleelectronic platform. The following discussion will focus on the specificsensor structure that is used to measure electrical transportproperties; the connections between the sensor array, the multiplexer,and the electronic platform, as well as their operations, are similar tothe connections and operations described above.

[0171]FIG. 18A show a preferred sensor structure that can measure theelectrical transport properties of a material. Like the above examples,a plurality of the sensors are disposed in an array format to measurethese properties for a materials library quickly. As is known in theart, resistivity is an intrinsic property that does not depend on thedimensions of the material, and resistance equals the resistivity ρ ofthe material times the length of a material sample L divided by thematerial sample's cross-sectional area A (R=ρL/A). The Hall coefficient,as is also known in the art, indicates the number of electrons or holesin a material per unit volume and indicates the sign of the chargecarriers.

[0172] Measuring these two characteristics according to the presentinvention is relatively simple. The materials to be tested are formedinto bars 180 having known dimensions, and the sensors used to test thematerial are equipped with six leads 182 that preferably contact the barof material at both ends and in the middle, as shown in FIG. 18A. Thebars 180 themselves can be formed by depositing the library elementsthrough a mask onto the leads 182, or by depositing the materials on thesubstrate first using a mask and then printing the leads 182 on top ofthe bars 180, again using a mask. In some cases, the bar 180 should besintered or annealed after deposition, so the anticipated effects of thesintering/annealing process on the contacts should also be consideredwhen selecting a depositing order. Like previous embodiments, thecontact pads in this embodiment provide the connection between thesensor array and the electronic platform that sends and reads signals toand from the individual sensors.

[0173] To obtain a material's resistance, an AC or DC current is simplypassed through the bar 180 and the AC or DC voltage across the bar 180is measured using leads EC or FD, without placing the sensor array in amagnetic field. To obtain a material's Hall coefficient andmagnetoresistance, the sensor array should be placed in a magnetic fieldB that points perpendicular to the substrate. The magnetic field B canbe generated in a variety of ways. For example, a large permanent magnetor electromagnet 184 can be used to generate a magnetic field that isperpendicular to the substrate over the entire sensor array, as shown inFIG. 18B. Any non-uniformity in the magnetic field B can be detectedbefore conducting the materials characterization, by using an array ofidentical, calibrated Hall effect sensors, and this non-uniformity canbe taken into account in any subsequent analysis of the data obtainedfrom the sensor array with samples. Alternatively, as shown in FIG. 18C,an array of permanent magnets or electromagnets 186 having the samearray format as the sensor array can be used in place of the singlemagnet. The pole portions of the magnet array are preferably placedclose to the individual samples and sensors in the sensor array. As forthe single magnet, the magnet array can be calibrated with an array ofHall effect sensors to detect any non-uniformities or variations in thefield strengths produced by the individual magnets so that thesevariations can be taken into account in subsequent analysis of the datafrom the sensor array.

[0174] During testing, a current is sent through the bar 180 usingcontacts A and B. The material's resistance in a magnetic field, knownas magnetoresistance, can be measured in the same manner as resistanceexcept the sensor array is exposed to a magnetic field so that thesensor can measure any changes in resistance in response to the magneticfield. The Hall voltage is obtained by measuring the voltage across thewidth of the bar, at contacts CD or EF, and the Hall resistance is givenby:

V _(H) =IR _(H) =I(B/nec)

[0175] As can be seen from the equation, the Hall voltage for a givenmagnetic field strength corresponds to the charge carrier density (n)and the sign of the carriers (+e or −e) for the material being tested.As is well known in the art, the Hall voltage results from the forces onmoving charge carriers in a magnetic field. This force, which isperpendicular to the direction of motion as well as the field direction,causes positive and negative charges to build up on opposing edges ofthe sample until the resulting electrical force on the moving chargecarriers exactly cancels the magnetic force. This condition can be usedto derive the above equation for the Hall resistance.

Experimental Example: Thermoelectric Material Properties

[0176] Yet another group of properties that can be measured using thesensor array of the present invention are two characteristics ofmaterials that pertain to their desirability for use in thermoelectriccooling devices: thermopower and thermal conductivity. Thermopower willbe discussed first.

[0177] When a temperature gradient is imposed on a material under opencircuit conditions, an electric field occurs due to the diffusion ofcharge carriers in the temperature gradient. In equilibrium, the forceon the charge carriers due to the electric field is just sufficient tocounteract their tendency to diffuse in the temperature gradient. Theratio between the temperature drop and the voltage drop across a sampleis known as the thermopower, S=ΔV/ΔT, and is typically measured in unitsof μV/K. The thermopower is a fundamental physical property of anelectronic material that can provide information about a materialselectronic structure and other characteristics. In addition, thethermopower is a key physical parameter for materials which are used asthermoelectric cooling devices. A large thermopower value is a desirableproperty for a material in cooling device applications. To search forimproved thermoelectric materials using combinatorial chemistrytechniques to synthesize libraries of thin films of candidate materials,it is desirable and necessary to be able to measure rapidly thethermopower of many materials.

[0178] The thermopower S can be measured using the sensor designexplained above and shown in FIG. 19A by measuring the voltage drop ΔVacross a bar sample 190 for a known temperature difference ΔT along thesample 190. This can be conducted in a variety of ways. In oneembodiment, as illustrated in FIG. 19B, a temperature gradient isimposed along the entire sensor array 10 by contacting two opposingedges 191, 192 of the array with metal blocks 194, 196 whosetemperatures are controlled and measured. If the substrate has highthermal conductivity, such as silicon, then heat losses to radiation andconvection will be relatively minor compared to the heat conductionalong the substrate, and a fairly uniform temperature gradient will beproduced. The gradient may be approximated as the total temperature dropdivided by the width of the array, and the temperature drop across anindividual sample will be the gradient times the length of the sample.In other words, ΔT_(sample)=ΔT_(array)*(L_(sample)/L_(array)). Moreprecise information about the temperature drop across each sample may beobtained by including two temperature sensors next to each sample withinthe sensor array 10, one near each end.

[0179] The above example ignores heat losses through the electricalcontacts to the sensor array, which may cause the temperature profile todeviate from the preferred linear gradient form or cause most of thetemperature drop to occur over a relatively small distance near the edgeof the array instead of evenly across the entire array. An alternativeembodiment which is not subject to this problem is shown in FIG. 19C. Inthis embodiment, a chain of heating/cooling elements 198, such asthermoelectric heat pumps, is used to impose a temperature drop acrosseach row in the sensor array, by means of blocks of metal or otherthermally conductive material that contacts both the heating/coolingelements 198 and the substrate. The elements 198 preferably alternatedirection so that all of the samples in the array 190 are at the sameaverage temperature. The structures that produce the temperaturegradient on the array may be integrated into the compression plate 40,shown in FIG. 4, used to apply pressure on the sensor array against thecontacts to the circuit board.

[0180] In yet another embodiment, the temperature gradient can beproduced by resistive heating elements that are part of the sensor arrayitself rather an external heating fixture. This structure is most easilyaccomplished if the substrate has low thermal conductance, either via alow thermal conductivity material (e.g., glass) or via a thin filmsubstrate (e.g., silicon nitride). A large number of configurations arepossible; ideally, temperature sensors are placed at both ends of eachsample and a resistive heating element is placed near one end of thesample. In addition, at least two electrical connections are at the endsof the sample for measuring the thermoelectric voltage.

[0181] The sensor array structure for measuring thermal conductivitywill now be discussed. Like thermopower, thermal conductivity isimportant for determining how efficient a given material will be for usein thermoelectric cooling devices. An ideal material in a cooling deviceapplication will have low thermal conductivity in conjunction with lowelectrical resistance to minimize heat leakage in the device and createa large temperature difference across the device with minimum energyconsumption and heat dissipation.

[0182]FIGS. 20A and 20B show a preferred sensor structure for measuringthe thermal conductivity of materials. As in the previous experimentalexamples explained above, the description will focus on the structure ofa single sensor, but it is understood that multiple sensors are used inthe present invention in an array format, and that sensors measuringdifferent properties can be included on the same sensor array. Also,analysis of thermal conductivity may be useful in materials researchcontexts other than the search for new thermoelectric materials.

[0183] The preferred thermal conductivity measurement method is viavapor-deposited films, on the order of half-micron thick, on membranes,similar to the structure used for heat capacity measurements. Othermethods may also be used to deposit thin film samples, such asevaporation from a solution or suspension. As in heat capacitymeasurements, thermal conductivity measurements preferably minimize theeffects of the substrate's thermal characteristics on the overallmeasurement results. FIGS. 20A and 20B illustrate a preferred sensorstructure 200 for measuring thermal conductivity. As can be seen in FIG.20A, the sensor structure 200 for thermal conductivity measurements canbe of similar construction and materials as the structure used in heatcapacity measurements, such as a silicon-nitride membrane, so that thethermal characteristics of the material sample can be easily detectedand separated from the thermal characteristics of the substrate on whichthe sample sits. Thus, the details of the structure will not be repeatedhere.

[0184] Referring to FIG. 20B, a desired sensor pattern is printed viaany known method, such as lithography, on the membrane 202 surfaceopposite the surface on which the material sample 204 will be deposited.This prevents a short-circuit from forming when characterizingelectrically conductive materials, such as metals. In this example, thesensor includes two wires 206, 208. The specific geometry of the sensorshould be optimized so that the temperature is substantially uniformalong the portion 205 of the sensor 200 over which the temperature willbe measured on the membrane 202 (e.g., the “active” portion). Toaccomplish this, the membranes 202 on the sensor array should be maderelatively long and narrow to insure that heat flow in the activeportion is predominantly between a second (heater) wire 208 and thenearby substrate 210, which contains a first wire 206, i.e., across thewidth of the membrane 202 (perpendicular to the heater wire 206) and notalong the length of the wire 208.

[0185] As noted above, a preferred sensor design includes two parallelwires 206, 208 having a known width and spaced a known distance apart.Branch leads 206 a, 208 a extend from each parallel wire 206, 208 andare spaced a known distance apart for conducting voltage measurements V1and V2 along the parallel wires. In this embodiment, the first wire 206is used as a thermometer and the second wire 208, which is on themembrane 202, is used as both a heater and a thermometer. As inpreviously described structures, the temperature is monitored bymeasuring the AC or DC voltage and current of the sensor and calculatingthe resistance, which varies linearly with respect to temperature.

[0186] In a preferred structure, the first wire 206 is disposed on thesolid silicon substrate 210, near the edge of the silicon nitridemembrane window 202, while the second wire s 208 is disposed on themembrane portion 202 of the substrate. The silicon in the substrate 210acts as a large heat sink to prevent the temperature detected by thesecond wire 208 from rising in response to the heat generated by thefirst wire. If the width of the membrane 202 is kept small (e.g., lessthan 1 mm wide and preferably less than 100 μm wide), heat losses due toradiation may be neglected in comparison to the total heat flow throughthe membrane and sample, and if the thermal conductivity measurementsare conducted in a vacuum, heat losses to the atmosphere due toconduction and convection may be neglected. Virtually all of the heatproduced by the first wire 206 conducts through the membrane 202 and thesample 204, in a direction perpendicular to the wires 206, 208.

[0187] The theory behind thermal conductivity measurements will now bedescribed with respect to the structure shown in FIGS. 20A and 20B. Asnoted above, thermal conductivity is a measure of how easily heattravels through a material when a temperature difference T2−T1 isimposed on a material sample. When a temperature difference ΔT=T2−T1 isimposed across a material sample, such as a bar sample, heat will flowfrom the warmer end of the sample to the cooler end. This heat flow J(in watts) is equal to the thermal conductance K multiplied by thetemperature difference ΔT. In other words, the amount of heat flowthrough the sample is proportional to the temperature difference acrossthe sample. The specific proportionality constant depends on both thesample's geometry and the thermal conductivity κ of the material,K=κ(A/L) where A is the cross-sectional area of the bar in the directionperpendicular to the heat flow, and L is the length. In this sensor,L=the distance from wire to the edge of membrane/substrate, andA=(thickness of membrane/sample)×(distance between branch leads).

[0188] Referring back to the sensor structure shown in FIG. 20A and 20B,the second wire 208, which is used as both the heater and thethermometer, carries a relatively large current I2 to generate a knownpower P for heating the sample and also measure the temperature of thewire; while the first wire 206 receives a small current I1 to conduct atemperature reading. The large current I2 should be large enough tocause significant self-heating in the portion of the sample around thefirst wire 206, on the order of 5 to 10 degrees C. The small current I1is preferably the smallest amount of current necessary to measureaccurately the resistance of the second wire; it should not be largeenough to heat the sample to any significant degree. Even though thesmall current I1 may cause the sample's temperature to rise a smallamount, on the order of a tenth or a hundredth of a degree, thistemperature change is negligible relative to the self-heating occurringon the portion of the sample on the membrane and can therefore beignored. Further, as noted above, the silicon substrate 210 acts as alarge heat sink, keeping the temperature of the sample in that areauniform and preventing the temperature of the first wire 206 from risingalong with the temperature of the second wire 208.

[0189] To measure the temperature difference ΔT=T2−T1, the electronicplatform only has to monitor the resistance changes in the two wires206, 208. The power I₂V₂ generated by the heater, which is equal to thetotal heat flux, and is input into the sample via the first wire, isknown from measurements of I and V. Because the geometry of the sampleis also known, the thermal conductivity of the material can be obtainedfrom the temperature difference. Note that the thermal conductivity ofthe membrane 202 still has to be subtracted from the thermalconductivity measurement obtained from the combined membrane and sample,to obtain the thermal conductivity of the material, but the membrane'sthermal conductivity is easily determined by sending current through thesensor without any material on it, i.e., before deposition of thematerial sample.

Experimental Example: Magnetic Material Characterization

[0190] The sensor array of the present invention can also characterizethe magnetic properties of materials libraries, again by changingpossibly only the sensor structure in the sensor array and making minorchanges in electronics and including equipment for generating a magneticfield as discussed with reference to transport properties. As explainedabove, the sensor array of the present invention can measure the Hallcoefficient of a material to determine the material's carrierconcentration and sign. In the present example, generally, an array ofunknown magnetic materials is placed on top of or in close proximity toan array of identically calibrated Hall effect sensors, which are madefrom a material with a known response to a magnetic field. An externalmagnetic field of variable strength is then imposed on the sample andsensor. The output of the Hall sensor is compared to the output of anidentical sensor that does not contain a sample. The difference in theresponse of the two sensors is due to the magnetization of the sample.In a preferred embodiment, the sensors with and without the sample areconnected in a differential arrangement, which greatly increases thesensitivity to the magnetization of the sample.

[0191] The samples may be deposited directly on a Hall sensor 210, asshown in FIG. 21A. In the illustrated structure, a sample 212 can bedeposited on one portion 214 of the sensor, with a second portion 216 ofthe sensor 210 left open to serve as a reference point. The differencebetween the voltages V1 and V2 when the sensor 210 is placed in amagnetic field corresponds to the magnetic properties of the sample 212.For example, the plot of the Hall voltage versus the magnetic field whenthere is no material on the sensor will be a straight line, but amagnetic material on the sensor 210 will cause the plot to deviate fromthe straight line, or will cause the straight line to have a differentslope, because the sensor 210 is measuring both the external field andthe field of the sample 212. In essence, the sensor 210 used in thisembodiment is a magnetic field sensor. Alternatively, the Hall sensorsand the samples may be contained on two separate substrates that arepressed together during the measurement. This later method allows reuseof the Hall sensor array.

[0192] Another specific way in which the magnetic properties of amaterial can be measured is by forming a sensor array containingcantilever sensors 220, as shown in FIG. 21B. A material sample 222 isplaced on a relatively soft, flexible cantilever 224, and then thesensor 220 is placed in a magnetic field 226 having a known fieldstrength and field gradient. The force and/or torque due to theinteraction of the field and field gradient with the permanent and/orinduced magnetization of the sample will cause the cantilever 224 todeflect. The amount of the deflection will depend on the strength of thesample material's magnetic characteristics.

[0193] There are several ways in which the deflection amount can bemeasured precisely. For example, the cantilever 224 on which the samplematerial 222 is placed can be one half of a sandwich capacitor such thatthe cantilever deflection results in a capacitance change. Analternative is to place the cantilever 224 on a piezoresistor 228, whichis shown in FIG. 21B, so that the bending of the cantilever 224 strainsthe resistor slightly, changing its resistance value. The electronicplatform can then monitor the amount the resistance changes andcorrelate the change with the amount of deflection. Other methods ofmeasuring the amount of deflection in the cantilever sensors 220 can beused without departing from the scope of the invention.

[0194] It should be understood that various alternatives to theembodiments of the invention described herein may be employed inpracticing the invention. It is intended that the following claimsdefine the scope of the invention and that the methods and apparatuswithin the scope of these claims and their equivalents be coveredthereby. The disclosures of all articles and references, includingpatent applications and publications, are incorporated herein byreference for all purposes.

What is claimed is:
 1. A method for characterizing one or more materialproperties for each of 5 or more samples, comprising the steps of:depositing the 5 or more samples on a substrate having 5 or more sensorsarranged in a sensor array, wherein each sensor is associated with oneof said 5 or more samples and characterizes at least one materialproperty of its associated sample; and measuring at least one materialproperty of said 5 or more samples at a rate of at least 1 sample every2 minutes.
 2. The method of claim 1, wherein the depositing stepincludes depositing the 5 or more samples on the sensor arraysimultaneously.
 3. The method of claim 2, wherein the sensor array has aformat compatible with combinatorial chemistry instrumentation, andwherein the depositing step is conducted by a combinatorial chemistryinstrument.
 4. The method of claim 2, wherein the depositing stepincludes: dissolving or suspending each sample in a solvent to form 5 ormore solutions or suspensions; placing each of the 5 or more solutionsor suspensions on one of the 5 or more sensors; and allowing the solventto evaporate to leave a film of each sample on its associated sensor. 5.The method of claim 1, wherein the depositing step includes placing atleast one material on each sensor by vapor deposition to create thesamples.
 6. The method of claim 5, wherein the vapor deposition methodis a combinatorial vapor deposition method that deposits two or morematerials in varying proportions on different sensors in the sensorarray.
 7. The method of claim 5, wherein the depositing step furtherincludes the step of heating the samples on the sensor array after theyare placed on the sensors by vapor deposition.
 8. The method of claim 2,wherein the depositing step includes the steps of: dissolving eachsample in a solvent to form 5 or more solutions or suspensions; dippingeach of the 5 or more sensors in one of the 5 or more solutions orsuspensions; and allowing the solvent to evaporate to leave a film ofeach sample on its associated sensor.
 9. The method of claim 1, whereinthe measuring step includes the steps of: changing an environment of asample to be characterized; and monitoring an electrical signal from thesensor corresponding to the sample to characterize said at least onematerial property of the sample.
 10. The method of claim 9, wherein theenvironment that is changed is at least one selected from the groupconsisting of humidity, temperature, pressure, illumination,irradiation, magnetic field and atmospheric composition.
 11. The methodof claim 9, wherein the measuring step further includes the steps of:(a) selecting one sensor in the sensor array; (b) monitoring anelectrical signal from the sensor selected in the selecting step; and(c) repeating steps (a) and (b).
 12. The method of claim 9, wherein themeasuring step comprises the steps of: (a) selecting a group of two ormore sensors in the sensor array; (b) monitoring electrical signals fromthe two or more sensors selected in the selecting step simultaneously;and (c) repeating steps (a) and (b).
 13. The method of claim 9, whereinthe measuring step comprises the steps of: selecting all of the sensorsin the sensor array; and monitoring electrical signals from the sensorsselected in the selecting step simultaneously.
 14. The method of claim1, wherein said measuring step measures more than one material propertyon the same sensor array.
 15. The method of claim 1, 2, or 3, whereinthe material property characterized in the measuring step is a thermalproperty selected from the group consisting of heat capacity, specificheat, thermal conductivity and thermal decomposition.
 16. The method ofclaim 15, wherein the measuring step includes the steps of: transmittingan input signal to at least one sensor for inputting power into thesample on the sensor; and monitoring an output signal corresponding tothe samples' temperature change in response to the input signal.
 17. Themethod of claim 16, further comprising the step of placing the sensorarray in a vacuum.
 18. The method of claim 16, wherein the input signaltransmitted in the transmitting step is a combination of a linear rampsignal and a modulated AC signal superimposed on the linear ramp signal,and wherein the monitoring step monitors a modulation amplitude in theoutput signal and an average value of the output signal.
 19. The methodof claim 18, wherein at least one sensor in the sensor array has aheater portion and a thermometer portion, the combined linear rampsignal and modulated AC signal is transmitted through the heaterportion, a DC signal is transmitted through the thermometer portion, andwherein the modulation amplitude in the output signal corresponds with aheat capacity of the sample and the average value of the output signalcorresponds with an average temperature of the sample.
 20. The method ofclaim 16, wherein the transmitting step transmits a linear ramp signaland an AC sinusoidal signal, and wherein the monitoring step monitors anoutput signal.
 21. The method of claim 20, wherein at least one sensorin the sensor array has a heater portion and a thermometer portion, thelinear ramp signal is transmitted through the heater portion and the ACsignal is transmitted through the thermometer portion.
 22. The method ofclaim 21, wherein a first frequency component of the output signalcorresponds with the average temperature of the sample and wherein asecond frequency component of the output signal corresponds with theheat capacity of the sample.
 23. The method of claim 16, wherein thetransmitting step includes transmitting an input signal that inputspower to the sample and wherein the monitoring step monitors the outputsignal to detect an increase in a modulation amplitude and monitors thetemperature at which the increase of the modulation amplitude occurs,which corresponds to a loss of mass in the sample.
 24. The method ofclaim 23, wherein the loss of mass in the sample is due to at least oneselected from the group consisting of decomposition, burning, andoutgassing of reaction products.
 25. The method of claim 23, furthercomprising the steps of: placing the sensor array in a closed chamber;increasing the temperature of the sample until it decomposes; andmeasuring the air pressure and mass of fragments from the sample insidethe closed chamber.
 26. The method of claim 15, wherein the sensors inthe sensor array are temperature sensors deposited on a top surface of asubstrate having poor thermal conductivity, and wherein the measuringstep includes the steps of: heating a first portion of the substrate;and measuring a temperature difference between two portions of thesubstrate, wherein the temperature difference corresponds to a heatcapacity of the sample.
 27. The method of claim 26, wherein themeasuring step measures a difference between the sample on the topsurface of the substrate and a bottom surface of the substrate, whereinthe temperature difference corresponds to the heat capacity of thesample.
 28. The method of claim 27, wherein the heating step comprisesthe step of increasing the temperature applied to the bottom surface ofthe substrate at a measured rate, and wherein the measuring stepcomprises the step of comparing the rate at which the sample temperatureincreases and the measured rate at the bottom surface of the substrate.29. The method of claim 26, wherein the measuring step measures adifference between the first portion of the substrate and a secondportion of the substrate, wherein the temperature difference correspondsto the heat capacity of the sample.
 30. The method of claim 29, whereinthe heating step comprises the step of increasing the temperatureapplied to the first portion of the substrate at a measured rate, andwherein the measuring step comprises the step of comparing the rate atwhich the sample temperature increases and the measured rate at thefirst portion of the substrate.
 31. The method of claim 1, 2 or 3,wherein the material property characterized in the measuring step is anelectrical property.
 32. The method of claim 31, wherein the sensors inthe sensor array are interdigitated electrodes, and wherein themeasuring step includes the steps of; transmitting an input signal to atleast one sensor; and measuring a response signal from said at least onesensor to determine a complex impedence of the sensor, which correspondsto the complex dielectric constant of the sample on the sensor.
 33. Themethod of claim 31, further comprising the step of measuring atemperature of the sample.
 34. The method of claim 33, wherein thetemperature and the complex impedance of the sensor are measuredsimultaneously.
 35. The method of claim 1, 2 or 3, wherein the materialproperty characterized in the measuring step is a mechanical property.36. The method of claim 35, wherein at least one sensor in the sensorarray is a mechanical resonator, wherein the depositing step includesdepositing a sample material on the mechanical resonator and whereinmeasuring step includes the step of transmitting an input signal to saidat least one sensor to operate the sensor in a resonance mode, andwherein the monitoring step includes the step of measuring a resonatorresponse.
 37. The method of claim 35, wherein at least one sensor in thesensor array is a mechanical resonator, wherein the depositing stepincludes depositing a sample material on the mechanical resonator, andwherein the measuring step includes the steps of: placing the sensorarray in a magnetic field; and generating a resonance signal in themechanical resonator; measuring an amount of damping in the resonancesignal, wherein the damping amount corresponds with the samplematerial's response to the magnetic field.
 38. The method of claim 35,wherein at least one sensor in the sensor array is a mechanicalactuator, wherein the depositing step includes depositing a samplematerial on the mechanical actuator and wherein the monitoring stepincludes the step of measuring an actuator response.
 39. The method ofclaim 35, wherein at least one sensor in the sensor array is amechanical actuator, wherein the depositing step includes depositing asample material on the mechanical actuator, and wherein the measuringstep includes the steps of: placing the sensor array in a magneticfield; measuring an amount of displacement in the mechanical actuator,wherein the displacement amount corresponds with the sample material'sresponse to the magnetic field.
 40. The method of claim 1, 2 or 3,wherein the material property characterized in the measuring step is anelectrical transport property.
 41. The method of claim 40, wherein themeasuring step includes the steps of: passing current through at leastone sample; and measuring a voltage across the sample to obtain theresistance of the sample.
 42. The method of claim 40, wherein themeasuring step includes the steps of: placing the sensor array in amagnetic field; passing current through at least one sample; andmeasuring one or more voltages across the sample to obtain either a Hallresistance, a magnetoresistance of the sample or both.
 43. The method ofclaim 40, wherein the measuring step includes the steps of: heating orcooling one portion of at least one sample; measuring a firsttemperature at the first portion of the sample and a second temperatureat a second portion of the sample; and calculating a temperaturedifference between the first temperature and the second temperature,wherein the temperature difference corresponds with a thermalconductivity of the sample.
 44. The method of claim 43, wherein theheating step includes placing a heater or cooler at one portion of thesensor array such that the sensor array has a heated or cooled portionand a non-heated or non-cooled portion.
 45. The method of claim 43,wherein the heating or cooling step includes placing a heater or coolerat each sensor such that each sensor has a heated or cooled portion anda non-heated or non-cooled portion.
 46. The method of claim 43, furthercomprising the step of placing the sensor array in a vacuum.
 47. Themethod of claim 40, further heating or cooling one portion of at leastone sample; determining a first temperature at the first portion of thesample and a second temperature at a second portion of the sample; andmeasuring a voltage difference across the sample, wherein the voltagedifference and the temperature difference corresponds with a thermopowerof the sample.
 48. The method of claim 1, 2 or 3, wherein the materialproperty characterized in the measuring step is a magnetic property. 49.The method of claim 48, wherein at least one sensor in the sensor arrayis a Hall effect sensor, and wherein the measuring step comprises thesteps of: placing the sensor array in a magnetic field; measuring aresponse of at least one Hall effect sensor; and comparing the responseof said at least one Hall effect sensor containing a sample with areference Hall effect sensor that does not contain a sample depositedthereon.
 50. The method of claim 48, wherein at least one sensor in thesensor array is a cantilever sensor, and wherein the measuring stepcomprises the steps of: placing the sensor array in a magnetic field;and measuring an electrical signal corresponding to said at least onecantilever sensor, wherein the electrical signal corresponds to adeflection amount of the cantilever sensor and the magnetic property ofthe sample material disposed on the cantilever sensor.
 51. The method ofclaim 10, wherein the environment is irradiation selected from the groupconsisting of ultraviolet, visible, infrared, gamma, electrons,neutrons, positrons, alpha rays, gamma rays, beta rays and x-rays. 52.The method of claim 16, wherein the transmitted signal comprises a stepor pulse and the measurement step comprises monitoring the temperaturechange of the sample in response to the stepper pulse, and determining athermal time constant.
 53. The method of claim 18, wherein a single wireacts as both the thermometer and heater.
 54. The method of claim 53,wherein the transmitting step transmits a linear ramp signal and an ACsinusoidal signal, and wherein the monitoring step monitors an outputsignal.
 55. The method of claim 54, wherein a first frequency componentof the output signal corresponds with the average temperature of thesample and wherein a second frequency component of the output signalcorresponds with the heat capacity of the sample.
 56. The method ofclaim 1, wherein said depositing is through automated sample dispensingor deposition.